Is this an idiom, a mistranslation, a reference to a cultural practice, or something else entirely?
The Codex Vaticanus text for this passage is:
ἔκχεον τοὺς ἄρτους σου ἐπὶ τὸν τάφον τῶν δικαίων καὶ μὴ δῷς τοῖς ἁμαρτωλοῖς.
Pour out your bread upon the grave of the righteous and do not give it to the sinners (my translation).
The Latin also says 'wine':
panem tuum et vinum super sepulturam iusti constitue et noli ex eo manducare et bibere cum peccatoribus (Biblia Sacra Vulgata, emphasis mine)1
Lay out thy bread, and thy wine upon the burial of a just man, and do not eat and drink thereof with the wicked (Douay-Rheims).
The NETS translators note at least three major Greek versions of the book of Tobit, the second (GII) is reflected in the text above. Be sure to read their introduction to the book for more information.
Codex Sinaiticus omits verses 7b-18, and the Aramaic has not been preserved. Some have posited that Tobit 4:3b-19 is an interpolation,2 while others believe its absence in Sinaiticus is likely accidental.3
But the practice referenced in this text appears to come from another contemporary text (and may have been cited verbatim):
... this is borrowed from the Proverbs of Ahiqar (Greenfield 1981: 332). The proverbs were not originally Jewish but were apparently taken over by the Jews at some point. This is probably why what was likely to have been a reference to a pagan rite is quoted in a Jewish writing.4
Interestingly, Tobit 1:22 mentions someone by the name of Αχιαχαρος (Achiacharos), which can be transliterated as Ahiqar:
καὶ ἠξίωσεν Αχιαχαρος περὶ ἐμοῦ, καὶ ἦλθον εἰς Νινευη. Αχιαχαρος δὲ ἦν ὁ οἰνοχόος καὶ ἐπὶ τοῦ δακτυλίου καὶ διοικητὴς καὶ ἐκλογιστής, καὶ κατέστησεν αὐτὸν ὁ Σαχερδονος ἐκ δευτέρας, ἦν δὲ ἐξάδελφός μου (Codex Vaticanus).
And Ahiquar asked about me, and I came to Ninevah. Now Ahiqar was cupbearer, over the signet [(i.e. 'keeper of the signet ring')], administrator, and accountant, and Esar-haddon [(Sacherdonos)], the second [in charge], appointed him, he was my nephew (my translation).
It should be noted that this follows the first Greek version noted by the NETS translators (GI), while the second (GII) also says that Ahiqar served under Sennachereim, king of the Assyrians, and then was appointed second by Sacherdonos.
The text says that Tobit's nephew is an official in the court of Esar-haddon at Ninevah, and this may be the same Ahiqar who composed the proverbs that were the source of v. 17 (or perhaps merely to whom they are attributed, cf. The Story of Ahiqar).
Jonas Greenfield (cited in the Eerdman's quote above) is the most notable scholar to note the relationship between Tobit and the Proverbs/Wisdom of Ahiqar. He presented this idea to the American Oriental Society in 1967, but I have been unable to find the actual article, which is fully cited in footnote #5 for your reference (and if you find it, please share!).5
1 The Vulgate numbers this as verse 18 of chapter 4.
2cf. Paul Deselaers. Das Buch Tobit (Univ.-Verlag; Göttingen: Vandenhoeck & Ruprecht, 1982), 50, 380-92. Also Merten Rabenau. Studien zum Buch Tobit (Berlin: Walter de Gruyter, 1994), 51-65, 149-50.
3 cf. Francis M. Macatangay. The Wisdom Instructions in the Book of Tobit (Berlin: Walter de Gruyter, 2011), 45ff, especially section 2.1.3.
4 James D.G. Dunn & John W. Rogerson. Eerdmans Commentary on the Bible (Cambridge, UK: Wm. B. Eerdmans Publishing Co., 2003), 741.
5 J.C. Greenfield. "Ahiqar in the Book of Tobit," in De la Torah au Messie: Melanges: Etudes d'exegese et d'hermeneutique bibliques offertes a Henri Gazelle (ed. J. Dore, P. Grelot, and M. Carrez; Paris, 1981), 329-36.