I think that the best wa to do it, is to define an engine and radio gear that you may be allowed to use. The engines and radio could be designed to show if it has been tampered with, to put better goodies in.
So, you could have a 2M (or smaller) airframe.
A standard cheapo, and maybe specially designed engine. Maybe a Thunder Tiger 120, or Irvine 150.
Standard exhausts.
Standard radio gear in the plane, something designed around 6kg torque, but no digital high speed coreless mini's. Maybe a price limit on the specific purpose designed servo, like say $15.

If there is alot of interest, then I am posotive that OS/YS/TT/Irvine/Futaba/JR would be more than willing to make gear fitting that criterea, and cheaply. (Remember, there won't be competition, so they will own the whole market, so they will sell thousands.)