I've actually done the opposite. I made my own face plates and then proceeded to warp them. (By mistake.)
The only reason I'm admitting to it is that I learned something by doing that (other than not to forget about them while they're in the oven) that could help you trying to straighten yours out.
I heated mine just to set the paint a bit more solidly. I had painted them on one side only. At not very hot (about 125 to 140F) they warped towards the paint. (It reminded me of film emulsions, where film and the old style photo paper bend towards the emulsion side.)
I just checked my original face plates, and they look as if they're painted on both sides, so my question above doesn't really ask much.
First thing I'd do is see if I could find a junk warped gauge to practice on. If the plate were warped concave up, I'd try the simplest thing I could think of; that would be to apply a coat of white paint (by hand is fine) to the convex side of the plate. I would place it in a 125 - 140F oven, paint up, on a 1 cm (1/2 inch) squared baking rack. I would put a metal or pyrex, open dish of water in the oven as well. I'd pull up a chair and watch it while it slowly bent towards the paint.
If I didn't have a practice shot, I'd feel confident enough to try this on the original. After all, I don't think it would really do much, if any harm. And it could work. It sure did with my albeit newer plastic.
The stainless wire rack(s) really worked well for me straightening out my warped plastic airbox cover. It's made from quite a different plastic than the gauge plates, and it is consistent throughout - cross-section wise (no paint or finishing differences on either side). So I think the pressure method worked because of that. And even then, I didn't really apply pressure while I was heating the cover - I just heated it up for 15 minutes or so, in a humid and 140F or so oven, and then took it out, put it between the racks, piled about 10 pounds of books on top, and let it dry out/cool down for a day. After that, even though it was flat, I did the same simple process again. That was 4 moths ago and the thing is still as flat as a flapjack. (Those Silpats are pretty neat, but they don't let stuff breathe at all. Plus, I wonder if pressure is needed at all, at the same time as the heat. If it is I would use one of those racks, mainly becasue they hardly touch the surface, yet they apply a pretty even pressure throughout.
The whole thing with the gauges I think, though, is that they do not have a consistent cross-section. And from my experience with warping my face plates, I really do think it's possible to create even more inconsistancy (by adding paint to one side) and then to use that difference to work for you by applying heat and bending towards the paint. (If the gauges are bent concave down, then I'd be tempted to try a clear, matte coat on the face itself, in order to warp it back "up" so to speak.)
Of course I don't gaurantee this procedure. It's all just an educated guess.
I will say though, that if no-one tries it out, the next time I'm at the salvage yard I'll procure a couple of warped face plates and see if I can work out a good way to straighten them out.
And I'm with Too Tired - let us know how you do.
Have no fear, and the best luck ....
