I used southwest engravers in okc, good work. I am not sure who is able to do this but I have seen some people engrave the underside of the lower near the trigger, that may be an option if you are worried about it.
http://ingunowners.com/forums/class-iii-nfa/139628-sbr-engraving-ident-marking-services-tx.html
http://ingunowners.com/forums/class-iii-nfa/139628-sbr-engraving-ident-marking-services-tx.html