We have Transponders but rarely use the Toll Roads any more since we moved to Irvine..
Once, we got a violation through the mail, my wife took her Car in for a service in San Juan Capistrano and drove home in a loaner, she used her transponder in it..
Anyway, I called the Toll Road People and they said the Transponder didn't match the Car - well, of course not, it was a Loaner!
They let me off the fine, but when I asked the Guy what to do in the future, he told me that next time we get a loaner/Rental Car to g online, register the Car and when we take it back, go in and register our own Car again...
To save all the hassle we don't use the Toll roads in other Cars with our transponder, we both lose, they lose money and we waste time sitting on the freeway ...
The Transponder is linked to our credit card, so why should it matter what Car we're driving?