I'm pretty sure all the MU frames except the frames in their 30 year classics are pretty much the same frame. I would think the old Mu Single would be a great frame to convert to Belt because of the horizontal dropouts---I'm not sure they came with that---but I'm not a Belt drive guy.
With my Mu D10 I just use an old sock to polish her up after a dirty ride. I do it on my front porch. Then a little bit of oil. I don't get why people think the derailleur bikes take so much maintenance. So simple, light and cheap to replace. I am assuming you are going with a 7 or 8 internal hub---lots heavier than 8-10 speed cog and derailleur. Good luck with your adventure. I just lined up some old Rolf wheels for my D10.. I'm going to try to lighten her up a bit. I am much fonder of 406 vs 451 wheels.