I've done grips on them. Magna-port does some customizing, with brakes and sights and polishing and so-forth. Gary Reeder used to do an Ultimate Backup on a fixed sight 83 that was quite similar to what FA will do now with shortened barrels and round butts.
Bottom line is, short of a few things that fall far short of "customs," there just ain't a lot to do to them, and a lot of gunsmiths are reluctant to work on them anyway. I would be very reluctant myself to do anything that might void the warranty - like let someone else screw up the trigger system, which does happen.