Makes me wonder if there's a way you could "hack" a standard wide format printer and swap out one of the ink cartridges, say black, for a pure white ink cartridge and then format your artwork so it thinks it's printing solid black, but it would lay down a white layer. Then you could swap the black ink back in and then print your normal CMYK. I guess technically you would likely do that in reverse so the ink layer is down and the protective film layer is up...