Congratulations, you've just reinvented Swerve Drive. I suspect you have already seen some robots like what you describe in action, and just didn't know it. Actually, its amazing the depth to which you have conceptually predicted designs wrought from tough trial and error. Now that you know its out there, I bet you'll have better lucking finding pictures of such systems on CD and the net. In particular i remember seeing a system using two gearboxes routed out to the four corners with horizontal chain, which sounds close to the design you were moving towards. To my knowlege no one has been using gears to switch directions, though. yet.
To get you started, these videos of 118 in action are what first got me excited about swerve drive:
http://chiefdelphi.com/forums/showth...ight=118+video