We seem to be getting variable road pricing over here in the Netherlands. Which generates a lot of discussion, of course. The Dutch ministry of transport has a nice high level overview including a diagram with some interfaces of the system:
I haven't made a detailed security analysis of this system, obviously. But couldn't one simply block the incoming GPS signal (say, using a GPS jammer). Better yet, why not relay the signal from a stationary GPS receiver at home to your on board unit?