A dichotomy twixt tools that shouldn't be there.  It looks like upas/fs always locks
L.mbox but that upas/deliver and upas/send lock the file L.(basename of mailbox being used).
Usually, they're the same but if we're delivering into other mailboxes, they aren't.
I'll make everything lock /mail/box/<user>/L.mbox and avoid the problem all together.
I'll run it locally for a while since I'm sure to generate at least one deadlock
along the way.