It is a tricky cryptarithm, but still solvable.
I've solved it, why don't you?
Well, this is an impossible cryptarithm in Base 10 or any other base, because M has to be zero and then U + S = the base but then U + S must = O - 1, which is a contradiction.
I'll bet a lira that I can make this work using Roman numerals. (Andy certainly left enough hints in that direction).
Gotta do errands, but I'll solve this later, analytically, unless somebody gets to it first. I'd love to see Charlie try to solve it with a computer program!
Nice puzzle, Andy!