Backdoor covenant - Normally like a regular coin, but an arbitary payload can be attached to it at any point in time through a transaction by the creator of the covenant. When spending it, there's a check for the lastest signed payload for it in the blockchain, and that is then executed.

Viral Proof-of-Work-Verified Payload Backdoor Covenant - Like the above, but here you are required to attach the covenant to another coin when spending this one, and *anybody* can attach a payload to it through proof-of-work, where the payload with the computationally hardest proof-of-work generated so far is the one who is executed when you spend the coin. The proof-of-work is of course individual for each instance of the VPoWVPBC.