My test mosaic piece finished at 15" x 19". I'm really glad I talked myself into a test drive. This would be a horrible, miserable thing the way I had imagined it. The iron on fuseable did not stay stuck down once the piece was being handled for quilting. Not all the pieces mind you, just every 12 pieces popped off. And the ones for which I no longer had a scrap of that fabric to replace it.
So I can cross that mental image off my to-do list and refocus on the dozens of UFOs I already have.