I've not secretly added Lisp code to the actual product, but I regularly do supporting work in Lisp: everything from personal tooling that doesn't affect everyone else to research items: prototyping, data analysis and whatnot. Mostly, I use TXR Lisp; a Lisp dialect I made myself that's good for this sort of use.

This is a great idea. I've often written small chunks of code in my favorite other language (scheme). I find this very satisfying.

Of course, I only do this after finishing the original task. Sometimes, it's possible to work up prototypes in your favorite other language before starting the main task. This often leads to better code (i.e. less code).

I'll bet that happened more than once. I always wondered how that Prolog core got embedded in the Windows NT network stack.

Did it. Wrote some Clojure in a large Java shop about 10 years ago. Took about 4 months before I got a slap on the wrist.