You could always just tack a C on the end of either, MV-22C or CV-22C, and be good with it.
Don't get me started, I think I ranted on this earlier in the thread but the whole designation scheme for the V-22 variants is bass ackwards to begin with. The logical thing would be to call it the CV-22C but that would likely make too much sense.
I thought a letter after the number meant a different series, not just different mission set.