I probably have no business answering, I haven't done anything like that since a Computer Graphics Cert in '95. But I did a little look at Uninet. I'm pretty sure you are out of luck, because "white toner transfer printing" is a big F'n deal. Some of my old classes were about how long it has taken to formulate the chemicals to create different shades, from the pigments Picasso used to paint... YAWN... modern laser jet inks. So, its not a matter of just putting a different set of inks, its a different set of inks that my all  work fundamentally differently. It's not just "white toner" (although for the printer operator it sure seems like it). I barely remember the Neugebauer equations: CMY creates 8 color combinations, with black, K, CMYK its 16, now add a white (W?), that's 32 color combinations (including blacks and whites: as shades or maybe they have another fancy name for it) except you have to imagine the cost of each of those inks, IOWs, how expensive it is per each page to translate the RGB screen colors into CMYK, except with white they've got twice the profit per page, because you've got CMYKW. It's not a matter of just a fifth more ink, its an entirely different process, the certainly could justify double profit if they wanted.

Or to look at it differently, if there was just a chance or this working with HP printers, they would have either bought outor partnered with  Uninet and/or at least put up quite a bit of competition for them. They ain't done either, looks like they won't.  So there's got to be a lot more value for Uninet printers and that method of printing.