As far as I can tell, D already implements this idea. The Rust/RustGC duality is very similar to D's approach, which has manual memory management and an optional GC [1], and DMDScript [2] could be the scripting language.
If we did SIL in C++ there's a good chance it'd have something like the UE4 header preprocessor to deal with all the hooks/metaprogramming we were able to D directly in D.