[SpeedFreed]

Well on first thought, you could have to pivot one mecanum wheel about axis of the other. While you wouldn't want to do this for anything extensive (I can't imagine mecanum wheels work well on significant angles) you should get a safe 1/8" of travel out of it, which would be plenty to account for any build tolerances. Then you just have to make sure the 4 solid mounted mecanum wheels are mounted such that they are coplanar (like any mecanum drive).

But, in my opinion, suspension is unnecessary complexity that should be avoided whenever possible. One of the goals in this idea was it's lack of articulation for simplicity and weight (and time and money...).