The menkyo kaiden is more likely to have been equated to 8th dan. The M-K gave a student a licence/obligation to teach off their own bat and possibly go forth and teach. 8th Dan was the rank given to the people sent out to spread aikido accross the world..
There's a longer list of "tenth dans" on aikidofaq..
It's hard to say, since they're completely different systems. Personally, I equate it to at least a 10th dan because it implies *final* certification - that everything in the tradition has been taught to and mastered by the student. That seems different to me than an 8th dan, after which there is the possibility of two more levels of promotion.