My guess is it will be like MinnowBooster's Lease Delegation Market.

It's not a true smart contract, but it's a trusted, market-based administrator.

So just like people are offering cards for sale on the SteemMonsters market, cards will be offered for lease on the SteemMonsters market. Probably on a per-day leasing basis.

Lessee puts the payment in escrow through SteemMonsters. Lessor grants leasing delegation authority to SteemMonsters. SteemMonsters administers the transactions with the agreed-upon timing.

That's how I would do it anyways. @aggroed and @yabapmatt might have something different in mind.