Here's what I decided, and have implemented for the next update:
There are now three levels of keys. Guest keys and master keys are unchanged from the current implementation. There's still only ever one master key for a home. But there are also "housekeys". A regular housekey will let you move furniture around, pay fees, and make guest keys and other housekeys. In fact, about the only thing you *can't* do with a housekey is change the locks. If the exile with the master key changes the locks, it will invalidate all the other keys. So... the master key serves the same purpose as a sort of "title deed", but people should be able to practically share their homes however they like.
Somebody commented that a scheme like this could still cause problems if the master key holder suddenly disappeared. That's still true, but I don't expect it to happen very often. Exiles should be careful to avoid such situations. In the case of something like an extravagant clan home, the master key holder should pass the key on to someone else if they know they're becoming less active.
As for the other changes...
* All homes should now be equivalent to the free library. Just /sleep in it, and you'll appear in the lobby when you log back on.
* Also, handtrucks are now useless. In order to pick up furniture, you /useitem your key (keys are now left-hand items so you can hold them along with a piece of furniture).
All other housing changes are secret, you know the drill...
Tinker