Wołu & bawołu?

Are wołu and bawołu the only two exmaples of a -u ending in the genetive and accusative singular of an animate masculine noun?

Probably not..

"Oczko mu sie odlepilo, temu misu" :)

I think it is the only case of both genetive and accusative with a -u ending and probably the only case where a masculine noun has an -u ending in accusative

I think they are exceptions. Traces of some old declination forms, I guess.

This is absolutely incorrect. The accusative of wół is woła. There is no masculine word in the Polish language that has -u in the Accusative, except foreign borrowings that end in -u and are not declined, ie. Bantu, Hutu, etc.

bawolu, wolu - dopelniacz
bawola, wola - biernik

There is also the word "gryf".
As mythical animal, it is "gryfa" in dopelniacz, but "gryf" as a part of the guitar, in dopelniacz it is "gryfu"

Iust to try to continue the fascinating issue I'll say "I don't know".
I hope yes.