Maclaren, I do see how you want to save weight, but as I look at it, I would like to see most of the weight put into the drive system. Sure, I may be bias because I drive the base but you need a solid drive system to get around a FIRST field now-a-days. Chains do weight quite a bit more then belts, but chains don't stretch typically, unlike belts that stretch more commonly. As Greg said, belts slip, chains dont, in the game of FIRST you would most commonly like to see the wheels slip before the drive system itself. However, I think it would be a cool endeavour but you may find out that it will give you periodic problems throughout the year. It may be cool to do in the fall, as a team building activity to train new team members.
On a bit of a sidenote, a different type of drivetrain done in the past was a
ball drive. It used metal pads to turn the balls, and chains to power the omnis. It was my teams "show bot" that was only used in a few matches during the 2003 season.