For a contract transferring property, you need 2 parties: One offering and the other accepting. Having knowledge of a cryptographic key implies none of that.
You could get something like this done by transferring the asset to a reputable trustee. The trustee - a law firm, bank, or such - checks that the paperwork is in order and it has the necessary rights. It binds itself contractually to convey some benefit - eg a revocable copyright license - to whoever can show that they have a certain cryptographic key/control of a wallet.
The firm should regularly check if the beneficiary still holds the key. It might get lost or forgotten, after all. The possibility of losing access to the asset by theft or accident is the only thing that involving NFTs add to such a scheme, so one might as well lean into it.