Can these contain other conditions, such as an expiration time to view them?
Pretty basic construction of Nostr silent DM's that break the DM graph privacy nightmare.
Others probably have suggested the same thing because it's too damn simple and obvious of a solution. Still gonna put it up here, maybe someone has comments. Might become a NIP proposal if there is interest.
I've coded it up and it works quite nicely.
https://void.cat/d/6gXGCYLJXqEeu4emNkM5TA.webp
Discussion
They are like ordinary DM's with the difference that the sender and receiver are not revealed publicly.