Maclaren, my old team 108 tried to use belts in 2003 and 2004 instead of chain simply because of the weight issue. 2004 was nothing but a huge mess. I have looked at the sketch you attached with your post. In theory it works (we had it done very similarly in 2004), but in reality it didn't. At the same time in 2003, team 108 used belts and pulleys and was very successful. It really depends on how you design the chassis.
The 2003 robot was a 4wd, and the whole front module would go back and forth horizontally in order to tension the belt (using a quarter inch 4" long bolt). On the other hand, the 2004 robot (4wd) had the tensioner tension the chain vertically which failed.
At this point, what I would suggest is design the chassis, post it up here on the forum and let Andy Baker, John V. Neun, Paul Copioli, Ian Mackenzie, Tim Baird have a look at it. There are many other engineers, mentors, and students who will help you perfect your design.
Here is a
picture of the 2004 chassis. I couldn't find a 2003 one, when I do, I will post it here.
Also when you get a chance, please read this
thread