White ink needs to be quite dense to work, which is difficult to get to work with water based ink sets and the piezo-electric inkjet printing technology. It likes to separate and clog print heads. There is the Epson WT7900 which ... basically has white ink. (Technically it fakes it.) It's around an extra $3,000 over the base 7900. I don't think they have a WT9900, but I could be wrong. So, interesting, but probably not useful for your purposes unless it's really really bugging you.
Interesting ink, however, Epson says short-term color stability and it's $8500. Here is what Epson says:
"Designed specifically for proofing flexographic and gravure print jobs that require the color white, the Epson Stylus Pro WT7900 incorporates our latest achievements in high-resolution ink jet technology - including our breakthrough Epson UltraChrome™ HDR White Ink.
Epson UltraChrome HDR White Ink introduces an all-new Organic Hollow Resin Particle Technology, which forces light to randomly scatter, producing the illusion of seeing the color white.
This innovative way to produce the color white yields an amazing white ink density, ability for custom white color toning, and provides professional short-term color stability for mission-critical color proofing on a range of new substrates including ink jet coated transparent and metallic films. And unlike traditional white ink chemistry, Epson UltraChrome HDR White Ink is a safer, water-based resin particle - void of any known carcinogens.
Capable of printing directly on both ink jet coated clear and metallic films, in roll or cut sheet, up to 24 inches wide, the Epson Stylus Pro WT7900 can proof virtually any size print job thrown at it. Whether you're creating contract proofs of various packaging products requiring a clear or metallic base, or producing color accurate packaging mock-ups or prototypes, the Epson Stylus Pro WT7900 can easily proof your most difficult flexographic and gravure print jobs.
With every new introduction of an Epson Stylus Pro printer, we continue to innovate and refine the overall experience. With the introduction of our latest Epson Stylus Pro WT7900 printer, Epson is once again redefining what's possible."