I've searched high and low for a waterproof medium that resembles charcoal, but to no avail. Fixative over the drawing repels my watercolors, so that doesn't seem to work either. BUT, I think I have a solution: photocopies. I draw with charcoal on a plain piece of paper, make a high resolution scan and print it out on watercolor paper with waterproof ink! Ha! I'll let you know if it works.
Another neat little plus with this is if I mess up with a painting, I just print another one out. AND I don't have to use my light table.