Jeff Smiley's method, The Balanced Embouchure, is what helped me figure out what was going on and to correct the situation. Mentioning it again because in this post over on Hornmatters, John Ericson talks about a high level horn player who used the method to deal with a blister in what sounds like the exact same place I had my callus.
Every so often I'll feel just the slightest ghost of the callus where it was on my lip, and then it will go away completely again. My best guess is that this happens when I let the mouthpiece get a little more dry than wet, but it's all so evanescent I can't be sure.
UPDATE - Dave Wilken was good enough to leave the following comment, which I'm bringing up to the post to increase the chance of someone looking for info on this subject seeing it. The comment reminds me that somewhere Farkas discusses dry and wet embouchures and says dry ones are more prone to having physical issues. I should also add that when I talked to my cousin, who's a pro level trombone player, about the callus, his suggestion was to just live with it. As Dave's saying something similar, makes me think trombone players are particularly devoted to their music ;-)
I used to get something very similar to what you're describing back when I played with a dry embouchure. It never really bothered me, but I do notice that they stopped coming back when I switched to a wet embouchure.
There's nothing wrong with either wet or dry, in fact I think it's good to practice a bit the opposite way you normally play to see what happens. Sometimes one or the other works much better for a particular player.
UPDATE #2 - Dave Wilken has expanded on this comment in this post over at his place.