Yes, that's true. Say you just edit the main file (i.e. 109g61.bmp). That's all of 1 file HTC has to build. You add another (G14CP.bmp) for the cockpit, and now all of a sudden, HTC has to build 2. You then have to add a lot of other things like the gunsight, the instruments, the faces, the needles, etc, and soon HTC has to build in 2 dozen or more files just for one skin.
Now, on the old planes, you'd have a side, a wing, a markings, and alpha for the markings, maybe an alpha for one of the side files. In the new ones all of these are eliminated into a single .BMP file. It's more efficient and takes up less space in the long run to put it all in one file. I bet from a programming standpoint it's easier to open and read 1 file rather than 5.
So, taking the cockpit, leaving it exactly as it is, but tacking on your on photoshopped "photograph" is basically a waste IMO. HTC will have to build the cockpit into the skin, just for the sake of a few different pixels.
You can try it if you like. I'm sure it'll get accepted, but unless you actually try to make some changes to the cockpit too, seems a bit of a waste.
Just my opinion.