No, not all Bluetooth enabled cars emit iBeacon transmissions. I can say this with certainty as my 2017 Mini Cooper pairs over Bluetooth yet does not emit an iBeacon transmission.
I also rent late model cars relatively frequently and am the developer of a number of beacon detector apps. While I have not explicitly tested all vehicles I have rented, I have never once noticed one triggering a detector app. My conclusion is that few if any vehicles emit iBeacon transmissions in general use.
I have worked with an insurance provider that provides beacons to be installed in customer cars (typically plugged in to a USB adapter) allowing vehicle usage tracking with the company app in exchange for a discount. An extra hardware solution like this would be required to accomplish your goal with iBeacon.