Willis,
You get the gold star!! Military experimental planes are something I had thought about, but I figured most would not be viable stunt planes. The only problem I can think of, offhand, is some sort of picture of the actual plane.
If you can convince the judges that it was an actual military airplane, go for it!!
In my research, I have seen bright yellow P51D's---who would'a thunk it??
(Target tugs---what a comedown!!)
Also, special colors, like the Navy Beetle Bomb Bearcat would be "legal", since the airplane was colored appropriate to its mission. Lots of possibilities there.
I plan to do a Pat Johnston P51D profile and have looked for unusual colors, like the target tug version. There were several foreign military P51D's with camo paint, bit I have not found a US or British camo D model.
Sounds like fun to me.
Jim