I have mentioned this before in this forum, but the best RC speed contests I ever saw were run here in the MidSouth several years ago. The contests were run by the Memphis Propbusters (my club) and the Whitehaven Taildraggers.

We did not use radar guns. Instead, we used a system in which there were two tripod mounted precision calibrated tracking arms. The tracking arms triggered a stopwatch which recorded the time. Of course, the setup was run over a 'closed' course. You could dive as much as you wanted before the 'trap', but your run through the trap had to be level, and judges were there to make sure

I think the idea of a 'proxy by internet' RC speed contest is a great idea, but you might want to consider having different classes by different displacements just like the Ukie guys do it.
(personally, I have 1/2A, 15size, and 40size planes that are well suited, and I imagine that other people have a wide variety of planes as well)

If you get any prizes for the contest, I think you better find a more consistent way to measure the speed than different Radar guns all around the country. The Speed trap idea would be much more accurate, but also difficult to make consistent all around the country/world.

Matchlessaero- AKA former Tortoise Award Winner at the 1990 Memphis Propbusters "Mr. Speed" Competition

Go Fast.......Turn ... Oops, don't turn, Just Go Fast