Hello!
Years ago I was using a similar technique for making my own printed circuit boards - I printed the layout of the board on clear sheet, then i have put the sheet with the "ink"/toner to the copper and by using hot iron I was able to transfer the toner to the copper. Then I didd the etching and the PCB was done! Actually if somebody would like to try this, one could probably make his own photoetch this way - although it would take some experimenting to get all the parameters straight.
Using some release agent between the clear sheet and the "ink"/toner sounds right for making custom dry transfers - I sure would love to see some pictures of how it turned out. Good luck with the process and have a nice day!
Paweł