There is already such a system in place for friendly NPCs. You can set a “zone” with the zone manager menu (default keybind ‘Y’) and set it to “No NPC Pickup”, which lets your allies know that you have a claim on the items there.
At some point we’d like to put in a system where NPCs can also claim items, and if you or another NPC picks it up they get angry (if they’re relatively friendly they’d just warn you that it’s their stuff, if not they might just start shooting). Actually we’d probably just extend the zone concept and mark terrain rather than individual items.
Likewise restricted areas that NPCs will be angry about you entering and similar things.
As for publicly announcing a claim, we were thinking of erecting some kind of totem that would potentially scare off hostile NPCs, and we’d probably want some kind of similar signage placed wen NPCs want you to not take their stuff.
To be clear, I’m totally against a df-like system where you magically know what is owned by who just by looking at it. There also might be unmarked territory that NPCs consider theirs, but they don’t announce it