I think you have covered everything. One optional rule that could be tried is a limit on number of terrain squares used ( e.g. 4). This will make the task more manageable in a competition timeframe and forces some compact thinking. Also can take into account any custom textures on existing vanilla objects and custom statics too.
Bfmods tshirts would be hysterical. I'll look into it.

I also have something else very exclusive that could be offered as a prize too actually. BF memorabilia....
Needs 2 parts to this - an ongoing catagorised competition and smaller time limited competitions where a theme, arena or other rules could be enforced. (even the 1 terrain square map etc!)
if I felt that I could put together a competition entry in a day or 2 it would be a lot more attractive than creating full maps.