I found the answer on another website: they use B8X, which in AA parlance is the angled widebody lead-in to a gate. Looking at some pictures of the terminal though, I think the eagle is hanging over the exit near B6, not over Legals.
I also found some pictures of the landing -- it's a pretty plane to have in Boston (even though we get a lot over in Terminal E).
New question: does AA have any other widebody-capable gates?
At one point earlier this year the 772 was being used regularly on MIA-BOS as well.