My team had a similar idea for the gearboxes this year, putting 3 CIMs on the outside of the drivebase.
Here's a few shots of the gearbox:
https://drive.google.com/folderview?...&usp=sha ring
In our experience, it works quite well for keeping the COG low and keeping the weight on the outside. The gearbox and CIMs are also very accessible. There's quite a bit of force on the inner gearbox plates though (the entire gearbox is essentially resting on them), just something to bear in mind.
Here's what our robot looked like at the end of the season:
http://www.chiefdelphi.com/media/photos/40089
Texas Torque did a similar gearbox last year, too. They used WCD. You can see a shot at 53s.
http://youtu.be/dSjHFgbcJZc