Follow-up, in case anyone's paying attention:
I took out the glass panel (not such a chore), used a small brush and white lithium grease, and lubed everything that looked lube-able. didn't see any damage. cycled it a few times, without glass. seemed to work? lubed some more, put the glass back in... seems to work?
I guess I don't really believe that what was broken before is now somehow fixed, but at least if it's not broken, it's not broken.
moral to the story: same as what's been said many times before: lube your d*mn sunroof.
recap the symptom: it would go back about halfway and do a nasty twist-jam-stop.