I have verbal confirmation from Alex Davy at Adepticon that all minis are intended to be set flat at all times (gradual slopes are considered to be flat). A speeder bike unit cannot balance partially on top of a barricade. The intent is to make play quick, smooth, and not have to argue if a mini unintentionally moved because it was leaning on something and the round base started to roll, or stop and try to balance a base for 30 seconds. Barricades are certainly not wide enough to do this.
I was the FFG contractor that was assisting Alex with judging duties for the duration of the weekend at Adepticon.