How about digital license plates? That change the code displayed daily. Not unlike authenticator phone apps.
Police can still use them to identify the vehicle, and verify registration, but mass surveillance and repo companies can't use them to track vehicles for more than a day OR to identify vehicles.
Flock uses other vehicle characteristics to track vehicles: bumper stickers, dents, etc.
The gov will have that database though and it's a simple query to get all the displayed IDs and then search them in Flock's database. It makes it slightly harder for Flock to do things of it's own accord but that's not the problem here.