Otherwise, if your design in mind would be implemented, which is for the "seller would get a code for the ATM and would have access to the compartment where the buyer deposited the cash", that would be more implausible, at least right now.
I would not frame it that much as an alternative to a current ATM. That kind of "automated cash locker" machine would be an alternative to in-person meetups, and some issues that can go wrong in that kind of meetup would be less likely (e.g. counterfeit money or the seller running away with the cash without transferring the BTC). Of course that needs trust in the machine vendor/operator.
I could imagine such a machine in hackerspaces, Bitcoin cafés or similar places.
The idea of an automated cash locker actually sounds interesting. I have issues, though.
1. The setup doesn't remove the custodian label. It's, after all, a machine that custodies money. The company is, therefore, offering custodial services.
2. Strictly speaking, it's a business that belongs to the financial sector. It accepts deposits from one person, keeps the money, and facilitates the payment of the same to another for a fee.
In which case, regulators might go heavy on it as far as AML/KYC/CFT policies are concerned since it's function is mainly to accept cash and dispenses it.
Perhaps an option is for P2P platforms to tie up with businesses that don't belong to the financial sector, say, coffee shops, concept stores, drop-off pick-up centers, and the like where you can either leave your cash payment or claim it. There doesn't have to be a meet-up. The store doesn't even have to know what the payment is for. KYC isn't necessary because they're just selling coffee, displaying all kinds of goods, accepting parcel deliveries, and the like.