Fantastic contributions from everyone, thank you all!
So, this is why I started this thread... I had no idea about these 3M products!
3M Hydrophilic Polyester Film 9962
3M Hydrophilic Polyester Film 9960
Game = changed!
Dave, I suspect that acrylic would be prone to the same issues as polyester, but IDK. You could try "roughing up" the surface with a fine sand paper if the gelatin comes off though.
I also like Umut's idea of etching the surface somehow. Ultimately, as hrst suggests, corona treatment, once properly sorted out, is a home-run as well.
Maybe the situation isn't as dire as I thought afterall. Though it would be nice to know if any of these aqueous ink-jet type PET films will take an emulsion well. It seems that if they will, then so should overhead transparency paper like Pictorico.