FediHood is a social network built around the place where you live. You choose a city and the topics you care about, then read what neighbours are saying and join in.
It stays a normal fediverse app. Your posts can reach Mastodon, PixelFed, Misskey and the rest, and posts from those servers reach you too.
Source code: https://git.holos.social/tom79/FediHood
There’s only one instance so far: https://fedihood.social/
If new instances are created, they should show up here: https://fedihood.fediverse.observer/list
key management is lot harder. signal does not support multi-clients for a reason, you can look at the history of matrix with encryption to find out what are the problem is. I’m not sure they even fixed or just know of all the failure modes of it.
Matrix is a good example I think: It allows a client to authorize another client so both work. It’s not the same ‘backup’ scenario, but proves the concept can work.
Is there an easy reference for the history there? I’ve used it over the years but not regularly enough to see it changing.