I am in New Jersey, do have a front license plate, and have an E-ZPass acount from the New York Service Center.
The following is the summary of my experiments:
Internal tag
1) To the right of the autopilot camera -- Unreliable
2) On the autopilot enclosure, above the mirror -- Did not work
3) On/under the pano roof -- Did not work
4) Under the trunk shelf -- Did not work
External tag
5) At the top of license plate -- Reliable, uber ugly
6) At the bottom of license plate -- Unreliable
7) On the grille, to the far side of the autopilot radar -- Unreliable
8) In the nosecone, above (not in front of) metal bumper -- Did not work