I don't know if this is something that is widely known in the printing community, but it shocked my printmaking professor when it worked.
Tools i used were:
I printed a 3 color, 18x24" print using these to make my film positives (2 colors done with this, and the key layer on rubylith). i had no problems exposing, wash-out was clean and crisp. Of course, the ink does feather a bit, but i'm sure a better vellum will help avoid this. You can also find these markers in varies nib sizes to vary your line weight. I really liked the fact that the positive is a direct hand-drawing and not a p.o.s.-scanned-lazer-print.
One important thing i noticed: They sell two different types of Poster Paint markers. I found that the water-based ones are much more opaque than the oil-based. only bad thing, the water-based ones are harder to find.
I need to find somewhere up here sells those posterpaint pens, they work good, and my red-o-pake and kimopake pens are too expensive. The students are always pressing too hard and plunging the tips = big mess.