Perhaps an appropriate OSS project would be to build a mill backend for LLVM. Since this would plug into all the existing front-end compilers for free, we’d be able to see how large real world programs would look in mill asm.
Or maybe start implementing a bunch of the interesting RosettaCode examples in mill asm by hand. I don’t know if RosettaCode would accept code samples in mill asm, but at the very least they could be listed and discussed here.
Of course, both of these would require some form of an asm spec. We may be able to construct effective mill asm even without the complete specification, i.e. omit everything NFY, but I wouldn’t know if it would be useful in that state.