From my expirience I never use a graver in plastic it tens to chip the line usually I done them with rotary and a steady hand I used a scm system the same as nsk and if you have diamond tips more control but remember my expirience my advise get a piece of the plastic and try it before doing the actual piece
I have three pistols with poly frames. One for competition and two for carry. They are tools only and any attempt to embellish them is akin to putting mag wheels on an armored personnel carrier. All of the engraving in the world will not take the ugly off of such guns. In my opinion (and I'm sure some will controvert it) putting your work on tactical guns will only harm your reputation down the line unless the "tacticool" crowd is who you wish to engrave for the rest of your career. Of course, if you are a hobbyist disregard everything I have written.
I have seen then for sale on gunbroker I think someone named white engraved them.
It's not the only thing I need to engrave, but for my area there is a market there's a void to fill, and I can fill it while still being able to do other work.
If you look at the pic search for engraved guns in Google you will find lots of tacticals engraved, mostly Mexico and the Middle East in origin. Lots of gold plated and made from karat gold coming from the same places. to each his own. One of the plantations here owners Mrs. shoots with a two million dollar pair of what could only be called a Maharajah Pair. Lots of gold sculpting and jewels galore. Her shooting coach, who travels with them showed them to me in the barn when I was resetting her horse when he lost a shoe while hunting. Rush job right away this instant or they would have a new shoer the next day. Fred