Not sure what the relationship with thin object pointers is.
There’s a contraption I did two years ago and @Diggsey has a brand new independent implementation and the only requirement is listing all the traits you want to expose ahead of time.
Removing that requirement ends up with polymorphic and cross-crate concerns, besides needing compiler support.
If you only generate the RTTI information only in executable crates then the cross-crate concern more or less go away and you can get all the monomorphic trait impls.
Polymorphic trait RTTI requires a much more complex system that can reflect on structural types and polymorphic nominal types (i.e. the whole typesystem) and effectively do trait impl selection at runtime.
It’s doable (Haskell and Julia are relevant examples, I believe), and Rust could do it without garbage collection, but it would be a lot of work for a slow system (if you don’t also throw a JIT in, which is even more work).