Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130722233052

I doubt Mayuri would attend a Captain Proficiency Test of free will, he would probably see it as a waste of time.

But he would probably know about his Bankai at least, as he told Ichigo that he knows about all Bankais in the Gotei 13. + Yamajii and the other captains who were present at Isshin's CPT.