CaptJack,
I have not tried the big one on the MPC - the 20" has worked so well (and frankly has enough room to make a meal for the neighborhood) I haven't felt the need. When I was last in ND I left "Roswell" with my brother (he was going camping) so, unfortunately, I can't try it right now. I would think it would work (the way the legs attach should not cause a problem) the only concern I would have is it is so big that it would take a while to get it to temp on the MPC.
I've used the smaller one (and there is one in between - 22") on live fires as well - just found some rocks or bricks and once some scrap metal, to prop it above the coals. So, legs aren't essential.
If I were going to do it again and just get one disk - I'd probably go for the 22" from Blue Moon (I like theirs because it comes with the bag and scraper - but I have seen no difference in performance between the two.) The 22" is probably (IMHO) a perfect size - big enough for a big group, but "small" enough to be easy to handle.
If you aren't in a hurry - I will be getting the big one back after Thanksgiving and will try it on the MPC and let you know how it goes.
Cheers,
David