It’s a huge project, I wonder if it wouldn’t be simpler to try to compile cranelift or mrustc to wasm (that’s still quite difficult) then use wasm2c to get a bootstrap compiler.
Edit to explain further: the point is for the code to be written (or at least auditable) by humans.
With your suggestion you can't use rust until after you already have rust-to-wasm transpiler available (which almost certainly itself already requires rust, so you are back where you started).