most machine shops would probably be able to bore out a small hole, which you could use to insert a small length of colored fiber optic cable... would be very custom and probably run you about 50 bucks... i can promise there is next to no one doing "custom parts" in your area for these little guns... however, with planning and asking around, alomost anything you could think of getting done should be able to be done with in a 4 hr radius of your local... google and the yellow pages are powerful tools when it comes to customisation...
im thinking about sending a set of grips to a local motorcycle shop with a CNC machine and getting billet aluminum grips made (they advertise being able to copy anything you can bring in on the machine!)