I remember reading your message at the tank combat site, and thought it was a cool idea. I've been to one of the games, and my impressions were:
1. Those guys have a lot more skill, time, patience than me in building complex mechanical devices, especially the scratch-built track drive systems.
2. The time and cost to build the tanks is high, prohibitive for most people.
3. Paintball guns add complications to the design - paint-proofing, heavy "armor", size weight, etc. Also restricts where you can have your games.
4. Too many restrictions on design - e.g. must be a model of an existing tank, must be a certain size.
5. I'd rather spend more time on advanced electronics than mechanics.
My thought was there should be a cheaper, easier way to have a similar kind of outdoor game that also allows for more emphasis on automation, sensors, etc, as in modern weapons systems. My first step was to build a 4-wheel drive vehicle, and this turned out to be a lot simpler than building a tank. Alternatively, people could use standard RC trucks, or maybe better yet, a rock crawler. An IR LED weapon would be smaller, cheaper, and lighter weight, allowing it to be used on an RC truck without having to modify it to carry more weight.
My 4-wheel drive vehicle uses 4 12-volt Denso right-angle gearhead seat motors ($17 each) attached to 8-inch lawnmower wheels. The rpm is about 160. These motors seem to be available everywhere for some reason. The control system is really simple. The motors are either on or off. LEft and right wheel pairs are independently controlled to get skid steering. The on-off controls are servos connected to Radio Shack 3-position toggle switches. The wiring is really simple. It works very well. The motor is about right for high torque, which is needed for the skid steering, but it's also pretty fast. The speed is more for paintball combat type games than for robotics. For a robot you would probably want less speed. The hard part of building it was attaching the wheels to the motors in a reliable way. I don't have a machine shop to make a decent hub attachment.