I suppose you can always try the tagged template approach, but then the other language is more like a pre-processor (edit: just noticed that this is exactly how the ClojureScript works in linked notebook at the start (waarom is Kees eigenlijk een topper, @jeroenvandijk?))
Take a look in the ganja.js topic for more ideas:
Also, you just inspired me to try embedding the Walt compiler in the Observable notebook, seems to work pretty well!
