I will have to defer that question to folks who use it. I use suction or muffler pressure.

As a SWAG, I would try a bladder before going to the trouble of modifying a backplate for pressure. See if it runs well at all on pressure. There should be info on the net for making and using bladders.

What vintage AME do you have? Before the REVLITE versions, the AME and Big Mig came with a brass needle (early) and steel/aluminum (later). The earlier version seems to have a finer thread than the later ones. This may be important when running pressure. The Zeus was made to run on pressure and has a fine thread.

Again, perhaps someone who has run pressure will respond.

George

Edit: Afterthought...you might want to check out some of the NORVEL info (15 pages), or ask the same question on the "1/2A & 1/8A Airplanes" forum. Good folks, very knowledgeable.