Does anyone use this in production? Will this be a good platform to develop a DSL for a web app?

Doesn't HN use Racket?

Edit: according to wikipedia Arc, which HN is written in, is developed in Racket

Is it in fact implemented in Racket? It used to be limited to PLT Scheme, which is the old name for Racket. But one of the changes that lead to the name change was making cons cells immutable. Arc was, at least at the time, mutating cons cells so Arc continued to use old versions of PLT Scheme instead of newer Racket.

I don't know if that situation with Arc has since changed though.