The Imagecast is twice as thick as the .001" poly. So if thickness is really an issue, that won't do it for you.
We have screen printed waterslide decals in the past but it's a real labor intensive job with 3 or 4 separate hits of clear along with the actual color prints. Unless your producing
a batch of them, it's hardly worth it.
I've thought about attaching the water slide
paper to a carrier sheet like Bill D. but have never followed through. If someone can get it to work I'd love to hear about it too.