I'd like a rule clause that says "If <some-particular-browser> is running".

It's a little strange, but my favourite browser is a script that opens a link in a new Chrome window.

I want to be able to say: "If Chrome is running, use my favourite browser".

Hmmm, after writing that (and sorting out in my head what I'd actually need from Choosy), I can see it might be a bit niche.