On the inside (guest) or outside (host)? On the host side, yeah if there isn't an RV version you should definitely write one (they do helpfully provide docs explicitly for that purpose). On the guest side, I seriously doubt that it's a good fit; RISC-V is designed for hardware and this is designed for emulation, which makes for different design choices.
I actually think Uxn and Varvara are a lot better suited to hardware than to emulation. RISC-V, however, includes a lot of concessions to hardware implementation that just add headaches to code generation and emulation, though not quite to the level of the MuP21.
