Just this past weekend had a long dress rehearsal one night and the two performances on the subsequent nights, and the callus shows no signs of reappearing, so I think I've figured out what was happening. Don't know how tuba and trombone embouchures work, but for me on the horn the issue was using too much of the outer visible lip in my buzzing and not enough of the soft inner lip. Using the tools involved in the Balanced Embouchure method (BE) also made it clear I wasn't using the larger muscles around the lips in the most efficient and natural way to produce my embouchure, so the buzzing parts of the lips probably didn't have the best support. Still don't know if the good tone I sometimes had will return, but my playing is more reliable, and there's no callus.
Valerie Wells, the horn rep for BE mentioned in an e-mail that she'd had a horn teacher with a callus in the middle of her upper lip that didn't seem to bother her. Also, Gabby, who plays baritone in the Friday group, had one briefly that went away, from what I understand, with the application of an ointment and making sure the mouthpiece was perfectly clean all the time.