I would say not renting software key but software key with some expiration date, for instance for 2 weeks.
It can be done in 2 ways:
1. There is a web application generating and selling this software keys for BTC.
2. A programmed smart contract which can get or generate software keys. But in this case you will get ETH instead of BTC.
The Bitcoin blockchain can be used to track ownership easily. nLockTime could also be used.
Maybe software on each user's computer that checks the blockchain for ownership.