Context virtually demands the future
Obviously if Isa 7:14 is taken primarily as a prophetic statement referring to Jesus Christ's birth (so Mt 1:23, being a prophetic message to "the house of David," v.13, not Ahaz himself, who did not want a sign, v.12), it would be future.1
However, even if taken as referring to the only other possible referent in the immediate context (as some commentators do), it must still be taken as a future. This is contra the argument you linked to, where Dennis Bratcher asserts:
All this says that much of Hebrew syntax, and therefore accurate
translation, is established by context not by the specific form of
words. In the context of the Isaiah passage, especially in the context
of the births of two other children in the immediately surrounding
passages, the grammar would best be translated as an English past or
perfect tense: "is [already] pregnant." This is followed by an
emphasis on imminent action, something that is "about to" take place
in the near future: "about to give birth."
The problem is he does not follow his own advice, because the only clearly contextual referent for those who do not hold this as a prophecy of Jesus is Maher-Shalal-Hash-Baz of Isaiah chapter 8.2 Of particular note is v.3:
וָאֶקְרַב֙ אֶל־הַנְּבִיאָ֔ה וַתַּ֖הַר וַתֵּ֣לֶד בֵּ֑ן וַיֹּ֤אמֶר
יְהוָה֙ אֵלַ֔י קְרָ֣א שְׁמ֔וֹ מַהֵ֥ר שָׁלָ֖ל חָ֥שׁ בַּֽז׃
The NKJV translation is good:
Then I went to the prophetess, and she conceived and bore a son. Then
the LORD said to me, “Call his name Maher-Shalal-Hash-Baz;
Though here, the NIV's less literal (and more dynamic) translation brings out more explicitly the euphemization that starts the verse:
Then I made love to the prophetess, and she conceived and gave birth
to a son. And the LORD said to me, “Name him Maher-Shalal-Hash-Baz.
Thus, if Maher-Shalal-Hash-Baz is the perceived referent, his conception was also yet future from Isa 7:14.
So while the grammar may be vague in Isa 7:14, the context is not vague (looked at from the perspective of either of the two most popular referents in the larger literary context of Scripture itself).
Given that even Bratcher in his article noted that the "'participle of the imminent future'" could mean "is going to" (rather than "about to"), having less imminence—even the Gen 6:13 and 17 parallels he notes are when God gives instructions to Noah to build the ark, so the whole time of beginning the ark construction (v.22), much less completing it, is between statement and actualization of the judgment. So too, the beginning (pregnancy) and birth (finish) of the Isa 7:14 prophecy can still be considered imminent even if over nine months away (i.e. prior to conception).
However, Bratcher struggles with the waw before the participle, stating:
So at this point we have to admit that we do not know the significance
of this connective
Yet a Christological view can be more emphatically maintained (my view, see note 1 below) when translated like so:
Behold, the pregnant young woman [or virgin], namely her giving
birth to a Son and she had called His name Immanuel.
Here "namely" is the translation of the waw Bratcher struggles with, being viewed as an explicative use clarifying exactly which young pregnant woman is in view.3 So namely, the woman who at the time of her son's birth already had (the verb for "called" is perfect tense) that Son named Immanuel. This views the first waw as the explicative to help qualify the woman being named, the participle as simply indicating a characteristic action (pregnant women characteristically give birth),4 but this characteristic action is accompanied by an antecedent circumstantial clause began with the second waw,5 noting the fact that she had already titled her son Immanuel prior to the birth (cf. Joseph's knowledge of this fact, Mt 1:23, and Mary's basis for understanding God with us by His act of conception with her, Lk 1:35).
So the whole thing may be viewed as a description of how to identify the particular young woman (even more so if a virgin) in question, who will bring forth the son that fulfills v.15-16. Note how this views the whole initial statement as merely descriptive of how to identify the woman (it does not indicate her future actions, but assumes since it is prophetic, that the hearers must behold and look for this sign that the Lord will give, which sign is v.14-16, and v.14 being only a reference to identifying the woman).
Whether Christ or Isaiah's son, both referents in the context of Scripture's revelation would be future. Only if one seeks to argue for a non-contextually referenced person could it be taken otherwise.
Postscript: Lack of Article on the Adjective
Hebrew normally has concord of definiteness between adjective and substantive in the attributive position, but there are exceptions (and Isa 7:14 appears to be one, see note).6 It is also true that a predicate adjective normally lacks the article, and that such an adjective can exist in a verbless construction, but it will normally precede the subject in this case.7
So here we have lack of concord but also not normal structure for a verbless predicate adjective. The grammar rules alone really do not help resolve the best translation, as exceptions can exist on both counts. Context still answers best, and with that pointing toward future, the typical translations placing it as a future conception are not bad in conveying meaning, but do not reflect the word as an adjective, but rather a verb.
1 For more on the future aspects of this prophecy, see what used to be the content of this footnote now part of this answer related to the fulfillment of the prophecy in Christ.
2 Fully unclear referents in the context would be a damsel from Ahaz's harem, or one standing nearby in the immediate context, and simply referred to by a hand motion. The commentaries discuss the unlikelihood of either, and the difficulties with seeing it as Isaiah's son as well.
3 Ronald J. Williams, Willams' Hebrew Syntax, 3rd. ed. (Toronto: University of Toronto Press, 2007), §434.
4 Ibid., §213.
5 An antecedent circumstantial clause "explains circumstances that precede the main clause. Its predicate is a perfect verb" (Ibid., §495a).
6 Ibid., §73. In Friedrich Wilhelm Gesenius, Gesenius’ Hebrew Grammar, eds. E. Kautzsch and Sir Arthur Ernest Cowley, 2d English ed (Oxford: Clarendon Press, 1910), it is noted in a few places that the article is ommitted for "euphony" (easing or making more pleasing the sound) with some of the gutturals, of which
ה is one (see §126.x), though it is my understanding that normally the vowel pointing and accent change on the article (which is itself a
ה). Both Williams (§82e) and Gensenius (§126.h) note that in poetry, the article is far more frequently ommitted—I have not explored whether Isa 7:14 is considered poetic in its form or not.
However, of particular note is another point of Gensenius' that relates directly to Isa 7:14. He states:
Peculiar to Hebrew is the employment of the article to denote a single
person or thing (primarily one which is as yet unknown, and therefore
not capable of being defined) as being present to the mind under given
circumstances. In such cases in English the indefinite article is mostly used [for translation]. (§126.q)
Note this easily fits Isa 7:14—being prophecy, it stands possible that the young woman is "yet unknown," but is here set forth "as being present to the mind under [the] given circumstances" of offering the prophecy, and it is certainly a "single person" being referenced. And in fact, Gensenius classifies the particular use here under that:
Is 7:14 (הָֽעַלְמָה, i.e. the particular maiden, through whom the
prophet’s announcement shall be fulfilled; we should say a maiden [cf.
Driver on 1 S 1:4, 6:8, 19:13]; Jb 9:31.
This probably best explains the lack of article on the adjective, as the articular use here is expressing this indefinite definitness (i.e. definite in that a single individual is in view, but indefinite, in that the individual is yet unknown). If Gesenius is correct (and I would tend to agree with him), this also further argues that (1) the referent is not someone standing nearby and known to Ahaz or Isaiah (the quality of being unknown is important here), and (2) infers a future pregnancy rather than a currently pregnant female, because the pregnancy and attendant circumstances noted are in fact what is going to identify and make this female known as the sign.
7 Ibid., §75.