LLVM backends are indeed modular, and the LLVM project does allow for experimental backends. Some of the custom optimization passes introduced by this MOS backend are also of broader interest for the project, especially the automated static allocation for provably non-reentrant functions, which might turn out to be highly applicable to GPU-targeting backends.

It would be interesting to also have a viable backend for the Z80 architecture, which also seems to have a highly interested community of potential maintainers.