It's in marks-menu.scm (create-loop-dialog), but, as you say, not fully functional. I think it's based on Richard Kent's work (am I remembering that right -- DAP?) -- I copied it in a monkey-see-monkey-do sort of mode, but I've never used loop points, so I don't know what is going on -- someone who cares about these things needs to tell me what they want -- or even better, write the rest of the callbacks themselves.