Whoa, check that! I decided to make a new profile and see where RT would save it. Turns out RT show us the file location for the default folder: ~/Library/Application Support/RawTherapee/profiles. I went there and, sure enough (despite my search not revealing it) there is an empty profiles folder there. I copied in my old profiles, relaunched RT, and there they all were, ready to use once again.
I'm not sure this profiles etc. location is what's specified in the RT developer's manual (if there's such a thing), but in my experience it's the right place for it in terms of most Mac OS applications.