Well, surely it would show up in the client, like a much stronger version of NIP-05 in nostr. Perhaps even, the client refuses to show the media unless it's signed, and the posting function has signing all rolled into it.

100% agree user friction kills it. How many people use pgp? However, https is pretty widespread and I certainly feel an emotional reaction when I have to go to a http site and my browser acts like I've got an incoming cruise missile.