Thanks David
In any case the cost-based stuff throttles I/O only (per the docs at least) but even while sleeping it still holds its lock. And it won't be kicked off of the lock by other processes. I don't see where it is documented that the autovacuum cost settings are altered during the anti-wraparound vacuum so I presume it will still sleep by default.
I knew I was misunderstanding something. I had picked up the impression that the vacuum process cost_delay released the lock for the period. Not just do nothing. Seems like that would be worth mentioning in the Docs. I'm learning this from the inside out in the tradition of "well someone has to do it". I'm sure I'm not alone.
-Senor