I built a system in my previous team where clients can register "filters" described in Fusion. My system, which was a source of a lot of different notifications, would then run these filters and only send those notifications that passed the filters. It became very popular very quickly because of the easy on-boarding and the fact that clients not get only a fraction of the messages they were interested in. I just checked the Java implementation, it seems to be still active and get commits.

Yeah, Fusion was the name. Last I heard, they discontinued it, saying essentially "If you really want a full Lisp, there's already Clojure." S-exps continued to be used in Ion for embedded 1-liners but they only supported a handful of operators, not a full language.

I hope you get an answer, because this sounds very intriguing but google is failing me in finding any references to it.

Dunno about that one but if you like that sort of thing, check out Rebol.