Going back to the Warhammer thing, this was what I was thinking about
That being said, I don't know how it will stand the test of time and wearing out in the field but it's something to start with/consider.
As for engraving, I guess one of those sports trophy/engraving stores could do it for you. You should have your "engraving" reference pic done in high res jpeg, svg, psd, postscript, or a similar "vector" image that is easily scalable and high resolution. Also it's probably best if you tell them it's for a RAP4 paintball gun or something if they ask.
Also I know this may sound really retarded but I'd actually consider putting a "Type R", "Type S", and "Safe" as my trigger group on a gun (corresponding to Auto, Semi, and Safe respectively).