Though I doubt it would work with mecanum, we used a new format of the "Tex-Coast Drive," (Grasshopper) essentially combining butterfly with a West Coast drive. We don't CAD nor do we have a CNC machine, so simplicity and ease of construction was something we needed. We built the first prototype and the competition drivetrain both in the same six week build season. It weighs just a bit more than a standard 8WD because of the pneumatic actuation. Here's a link to what I mean:
Grasshopper Drive System