Dirk,
I did find this webpage which offers a white decal paper:
http://www.papilio.com/white inkjet waterslide decal transfer paper media.html
Also:
CrystalTM UGW â¤" White UV Inkjet Inks for Display Graphics
A key requirement for the display graphics inkjet market is white ink.
SunJet now announces Crystal UGW white inks. These inks have been
formulated to offer printers the ability to choose where to put white
ink when printing onto transparent or dark media substrates. The new
white inks have outstanding opacity and whiteness unprecedented in
inkjet technology....
(for full info go to this URL):
http://www.sunchemical.com/press_2003_1_1090856727441.htm
White Ink on the Fast T-Jetâ"¢ Pro-HV!
After six months of R&D and testing, we now have white ink on our high
volume Fast T-Jetâ"¢ Pro-HV. Imagine printing 100 to 150 shirts perhour
with bright white and colorful prints! For more information check out
the Fast T-Jetâ"¢ Pro-HV product page and look at the video of it
printing white ink.
It is a thing of beauty!
For the full text and link, go to the following URL and look down the
page about half way on the left side.