Kodak used to make a red opaque mask that came in a small bottle. I have one and I think it's pretty dried out but if it's like Spotone, a bit of water on a brush to grab a bit should work. I'll see if I can dig it out and get the info...

ok, just went down to the cave. It's called Kodak Opaque Red. I also have the Opaque Black. Not sure where you could find that particular item but I imagine a good art supply or graphic arts store would have a clue as something similar was used heavily in pre-press work I think.