! Would love to find out :) It's a open goal to make ClojureScript pluggable.

I'm by no means a Lisper or "compiler guy", but I found the ClojureScript compiler to be extremely hackable. It looks like it would form an excellent basis for Clojure-in-Clojure, which would surely support additional pluggable backends. Seems like that is the plan.

I think the biggest problem would be writing a garbage collector. It would be huge, though...