Your bulldozer would need to have a chip installed that can connect to Ethereum and read off who the current authorized operator is. The operator and the owner would have to have cards that contain keys that speak a protocol that the bulldozer understands. Once the loan time runs out, the bulldozer locks out the renter.

The GP is describing a system kind of like Zipcar - you go online, you enter payment, and Zipcar allows your card to operate a car for a period of time. So, you might say, so what, we already have a Zipcar, why replace it with the blockchain?

Because if Zipcar goes out of business, all the cards and hardware on Zipcar cars are defunct. If all the hardware talks to contracts on Ethereum, there's no Zipcar to go out of business - you buy the hardware once, but the operation of the system happens on Ethereum, not in Zipcars' SQL database.