I've washed cars that were on jackstands (or otherwise immobile) plenty of times. I'd cover the brake rotors so they don't get all rusty, but other than that there shouldn't be any problems.
I wouldn't just QD it because more marring is more marring and that'll end up meaning less paint.
But, speaking as a guy whose RX-7 seems to *live* on jackstands (and never mind the XJS

) I'd just wait until you get it down. If you'd normally pull it outside to wash, it sounds like your garage might not be the optimal place to do that job anyhow (e.g., where'll all the water go?).