I mustn't be explaining myself very well

I'll try and address these:
@Brigit - this isn't to replace handover of land to groups. That's a special case where the person wants to "fully" own the land, I don't expect to see scripted systems capable of this in the future.
I'll do this on the basis of what I would like to do:
- Imagine I have a simultator, it has a load of nice little houses connected by streets. Essentially a perfect residential area
- I want people to be able to rent a home, and place any items they want inside it, allow anyone they like inside etc.
- I don't want other people's objects littering the residential area.
Current solution would be something like:
- Each house is on its own plot but is owned by the same group (e.g Haravikk's Rentals)
- Each plot has auto-return enabled so non-group items are removed
- When a renter pays my scripted object to rent the house, they are told to join the above group
- Now they can place their items
This is not ideal however, as people who are not renting with me could join the group and drop garbage about, or people who stop renting will not have their items automatically returned, instead the house they were in would have to remain closed while waiting for someone from my rental group to come along and clean it up. Also is the issue that ANYONE in the group can place items on ANY parcel, meaning your neighbours could fire weapons on their land and leave your garden full of shell casings or whatever without them being cleaned up.
Alternatively I'd have to make the group invite-only, in which case renters would have to wait before they can start placing items.
With the Allowed List being used, the renter, and people of their choosing (via the rent script) can be allowed to place items by the script (it just adds them to the parcel's allowed/access list). This way they don't have to be invited into a group, or have the security hole of an open group. So the system would run like so:
- Each house is on its own plot and owned by the same group (Haravikk's Rentals)
- Each plot has auto-return enabled so non-group or allowed list items are removed
- Renters pay the rental script in order to purchase that house
- Rental script adds them to the Allowed List for that house's parcel
- Renter can now place items
Now any trash that is placed is removed unless the user is part of the management group, or is on the allowed list for that house/room/plot. Also, if the renter stops paying, or leaves, then the rent script just removes them from the allowed list, and their stuff is returned. It allows for the creation of a very tightly run rental system which prevents trash lying around at high altitudes sapping people's prim-count and such. It also allows people to be granted access to a house/room on a per house/room basis, similarly with creating items. So just because someone owns a house somewhere else on my island, doesn't mean they can litter in other houses.
Access to the house would of course still be up to the script in this example.
@Grumble - those new features are going to be great for managing scripted rentals. But they only really help a bit with managing objects themselves.
In essence this suggestion would allow for a kind of llReturnObject(key id) system. Instead of returning specific objects, you set-up auto-return, then via a script allow people to place objects, whether permanently or temporarily is up to you.