What is the significance of what Jesus ate after his resurrection? Is anyone aware of any special symbolic significance of Jesus eating specifically fish and honeycomb while showing his disciples he was not "a spirit" when he appeared to them (and somewhat scared them) after his resurrection in Luke 24:42-43?
Some commentators do employ a bit of speculation on this question, including Augustine himself.  The Ancient Christian Commentary on Scripture summarizes Augustine, saying that the fish "represent the faith of the martyrs that have gone through the fiery trials of suffering." With respect to a similar passage in John 21, Augustine writes that "[t]he fish roasted is Christ having suffered" (Treatise 123).

With respect to the honeycomb (which some manuscripts do not mention), Jamieson, Fausset, and Brown write that it is simply "common frugal fare" (Commentary).  Matthew Henry likewise calls it "mean" (in the sense of "humble") and suggests that a lesson can be drawn from its inclusion:


  The honey-comb, perhaps, was used as sauce to the broiled fish, for Canaan was a land flowing with honey. This was mean fare; yet, if it be the fare of the disciples, their Master will fare as they do, because in the kingdom of our Father they shall fare as he does, shall eat and drink with him in his kingdom. (Commentary)


The more important point, attested by both church fathers and more modern commentators, is that Jesus is proving to his disciples that he is truly resurrected and that he has a real body.  As John Calvin writes:


  During the whole course of his life, he had subjected himself to the necessity of eating and drinking; and now, though relieved from that necessity, he eats for the purpose of convincing his disciples of the certainty of his resurrection.