PJ/RM, I haven't tried the method you're talking about, but it has crossed my mind. I do know that Elvgren used photos and did light sketches on his canvas before laying on the paint. FWIW, coloring books use a similar method as well - lines on paper. Likewise paint-by-number sets. I see no reason why your plan wouldn't work. You would only have to put the drawing onto a surface appropriate for your chosen medium - watercolor/acrylic paper, oil paper, and the aforementioned sheet plastic. I would lightly scuff the plastic and primer it before painting.
My method is similar. I take a photo - or photos - and scale them in Photoshop to about twice the size I plan on sculpting and painting. I trace the photo onto vellum with a 2H pencil, making whatever changes in arms, legs, hands, hair, expression, etc., I need. Once that's done, I can redress the figure in whatever clothing (or lack of) I want. Once finished, I ink the drawing, and scan it to my chosen scale, which is about 90mm it seems. The torsos (the Elf pinup) are made at 90mm from bottom to the top of the head. Hats are extra (Steampunk pinup). I've pretty much settled into the 90mm size, but some earlier attempts were larger (Texas Heat and Bathtime Surprise). After inking, I put the drawing onto a small light box (used for 35mm slides) and trace the drawing onto a piece of .020" thick white plastic (Evergreen of Plastruct). I use sharp scissors - straight and curved - to cut the piece out. At this point, it's ready for putty. The basic drawing doesn't take much time at all, but my drawing skills aren't such that I can whip through the changes. Lots of erasing and do-overs. I can go from photo(s) to plastic in a couple of hours or so.
Cheers,
Glen
