Yay, it works!
Build recipe to convert box wine into a quick and easy blood bag:
1 press-clamp style pants hanger, (example: 'Bumerang' hanger from ikea, 99 cents)
8" of duct tape
1 chopstick (or dowel, shish kabob skewer, whatever)
1) Remove the bag of wine from the box
2) Place the chopstick on the duct tape and carefully roll the tape over the top edge of the wine bag.
3) Clamp the pants hanger onto the bag, putting the chopstick just above the clamp. The chopstick is to keep the wine bag from pulling free of the clamp on the pants hanger. It will slide down until it fetches up against the chopstick.
4) Make a sign saying "Whole Blood" (for red) or "Blood Plasma" (for white) and tape it to the bag.
5) Hang the bag from something convenient (I used a microphone stand)
I wish I could figure out how to attach tubing to the bag valve so it dispenses through the tube, but oh well. I'll just hang some tubing on the stand for looks.