Hi Gavin - I am very impressed with your work and enjoy the photos very, very much. Also, wasn't it you who came up with that out of this world backdrop method on the Gauge??? THAT Sir, is simply the best advice I've ever gotten.
I had a thought on the white lettering for your decals. Give this a try and let the Gaugers know if it works. This is an experiment.
On the computer, white is actually comrprised of three colors. Red, Blue and Green. Each color has a value of 255 (full on) and that makes white. It is also represented by Hue (base color), Saturation (how much) and Luminence (brightness). Since white is also the lightest value in gray scale, those settings are 160, 0 and 240. One more value (Hex) of white is #FFFFFF
Might I suggest you try setting the color of your text to a very, very light gray by changing the values to:
Red: 253
Blue: 253
Green: 253
or
Hue: 160
Saturation: 0
Luminence: 239
or
#FDFDFD
I don't know for sure, but I'd bet the printer will accept that as a color other than white and may actually print it. Give it a try.
(I would, but I don't have any paper that's not white)
Hope it works!