Last night as I lay in bed with total insomnia trying to solve the worlds problems, or atleast my own, a lightbulb went off. In my brain anyways, it occurred to me, if you heat set the crayon drawing first, the wax would melt into the fabric creating a resist.
I just did a quick little test and lo and behold it works! Draw your design on the fabric, put a press cloth on top and iron it at the setting for the fabric. Then use transparent textile paints with a lot of water mixed in as you paint. You could do lots of watercolory blending of colors and throw salt on it to get more texture as it dries.