Given how close SubX is to x86 -- and Mu is to SubX -- I'd say 'porting' is the wrong word. So writing something in the spirit of SubX/Mu for RISC-V would be a non-trivial effort. Still not that much, since there isn't much code, and since there's lots of scope for manually transliterating existing code piecemeal.
If someone started this I'd be glad to contribute to it.
