I highly suspect it won't be feasible for the same reason it isn't feasible in C++: you could technically implement it, but tons of existing patterns in the ecosystem would become impossible to express, so in practice it would end up creating a different language. From a skim, the CLR project you linked to claims that metadata will probably be needed in order to enforce aliasable xor mutable, and I agree.
> tons of existing patterns in the [C/C++] ecosystem would become impossible to express
Well, the really harsh way of putting this is that the patterns break for a reason; they rely on global claims about the program, so they aren't genuinely robust in the context of code that sits within a large, constantly evolving codebase that can't be practically surveyed in its entirety. Rust is very good at picking patterns that can be verified with a comparatively straightforward, "local" analysis that broadly follows the same structure as the actual program syntax. Safety claims that rely on "global" properties which cannot be kept within a self-contained, module-like portion of the code are essentially what the unsafe marker is intended for. And this is exactly what idiomatic C/C++ code often gives you.
This is actually why I think that proposals like Safe C++ should get a lot more attention that they do at present. Yes, Safe C++ changes what's idiomatic in the language but it does so in a way that's broadly sensible (given our increased attention to memory safety) especially in a context of "programming in the large".
you can go a long way before getting to aliasable xor mutable, and the metadata doesn't require a language change, theres an example in there on how to bind metadata with no language changes.
