I just reread the EZPass brochure, as I'm trying to buy it from other states (MD started charging a monthly fee) and it said that the transponder was designed to be mounted at an angle. (as in the windshield angle). I used it on my tankbag and it was hitandmiss. I now habe it on the fairing.
Here in the bay area, they'll give you a big fat ticket for using your e-z pass on a vehicle that the E-Z pass isn't registered with (my parents got a huge fine for this when they had a rental car)
This made me realize they're doing a scan of the plates automatically, so I don't even bother to take the pass with me on the bike.
One less thing to carry, and more work for the toll collectors