Is 'many' necessarily exclusionary?
No. The relevant bit of Heb 9:28 (NA-28 | ESV):
οὕτως καὶ ὁ Χριστὸς ἅπαξ προσενεχθεὶς εἰς τὸ πολλῶν ἀνενεγκεῖν ἁμαρτίας
so Christ also, having been offered once to bear the sins of many
The contrast is made between Christ's once offering (prospherō) and the many whose sin he has taken upon himself (anapherō). This once/many dyad is reinforced by the repetition of the root pherō in these two verbs.1 The emphasis is not on the lack of a form of the word πᾶς ("all", which would be a rather odd emphasis to go unstated in any case), but on the contrast between the singularity of the offering and the multiplicity of those whose sin was borne.2
It's also worth keeping in mind, as noted in just about every list of cross-references, that Heb 9:28 is a reference to Isa 53:12, LXX (Rahlfs | NETS):
διὰ τοῦτο αὐτὸς κληρονομήσει πολλοὺς
Therefore he shall inherit many,
καὶ τῶν ἰσχυρῶν μεριεῖ σκῦλα,
and he shall divide the spoils of the strong,
ἀνθ᾿ ὧν παρεδόθη εἰς θάνατον ἡ ψυχὴ αὐτοῦ,
because his soul was given over to death,
καὶ ἐν τοῖς ἀνόμοις ἐλογίσθη·
and he was reckoned among the lawless,
καὶ αὐτὸς ἁμαρτίας πολλῶν ἀνήνεγκεν
and he bore the sins of many,
καὶ διὰ τὰς ἁμαρτίας αὐτῶν παρεδόθη.
and because of their sins he was given over.
Here again, the idea is that one individual (on whom, see the preceding verses) shall inherit many and has borne the sins of many. The verse is silent on the issue of whether he has borne the sins of all; that's not the point.
1. Despite the fact that neither of them looks like pherō nor like each other, this connection would be obvious to the Greek reader.
2. See also: Heb. 2:10; Mk. 10:45, 14:24.