We're going with mecanums, but with a twist... one pair is direct drive, while the other is chained. Pic of our frame:
Basically, The back wheels are direct driven, and we cut out holes for 1/2 bearings to fully support the wheel, centered on one of the smaller holes along the bottom. The other wheels will be run on a dead shaft in the front, chained to the motors that are in the middle. They are all run using KoP gear boxes (we had two we never used last year and guess what? Same gear reduction!). We're avoiding cutting out any of the flanges on the frame, and instead are building a mount for the gearbox that will stand it off .625" (i think), which is the width of the flange. This also required turning custom axles, as the normal KoP output shaft isn't long enough once we move the gear boxes off the frame like that.
The 6" white wheels in the pic were just used to ensure proper clearances with the front and back frame members... the mecanum wheels aren't in the shop yet.