Huertecilla,
This sounds like a fine idea and I think you should be able to pull it off with no problem.
There are all sorts of different materials that have been used as LCD reflectors and they all seem to work reasonably well. Some watches use silvery plastic foil, some use semi transparent reflective foil with a backlight underneath, some use textured metal reflectors (silver or gold), some use colored plastic or paper (usually green).
Below is an example of a Tritium backlight mod that I did. I wore this watch to a film festival this weekend - works great in the dark. The tritium actually uses a phosphor to create its colored glow. The tritium itself just emits beta rays, which don't glow unless they strike a phosporescent material. Hence, the different colors of Tritium vials are actually due to different phosphors:
http://www.timetrafficker.com/watches/l ... index.html
Tritium is rather pricey, though, so if you find a decent phosphor / superluminova type solution that works well, then that would be a good alternative.
I look forward to some pix of the phosphorescent backlight. If it works, maybe you can even try different colors!
-abe.