There was a .tga file used for the 7c deck light map (LM?) that didn't appear anywhere in the Textures folders, either of GWX or any of Aces' mods. It was just embedded in the 7c .dat file. Anyway I exported it and tinkered with it a bit, and imported it back in. That resulted in the decking under the (potential) deck gun being displayed properly with the AO map enabled in the model preview. I'm about to stick it in the game and see what happens.
Edit: SUCCESS!!!
Now it won't matter whether or not the game fits a 7C with a gun or not, or whether I add or remove one or not. No doubt I will lose the shadow effect on the deck of having one if I actually do have one, but that's a small price to pay IMO.