This is fairly self-explanatory. It is quite simply an option somewhere on the object tab next to auto-return, which allows us to exempt users entered into the allowed list for a parcel from having their items auto-returned.

Imagine the following scenario (and it's the reason I would like this):
I am renting out an apartment space which is located on its own little plot of land. To keep the area tidy I wish to enable auto-return, however this presents a number of problems. Namely, if someone rents a room in the apartments, then they must be part of the group set on the land in order to place items within it, otherwise they will be auto-returned.
However, adding them to the group manually can take time, time in which they want to be using their room. The alternative is to have the group have open-enrolment, but this prevents further problems as it means that anyone can join it then litter the apartments anyway.

This suggestion however would enable the allowed-users list on plots of land to be used for this purpose. So the system would work like:

• User pays rental box L$ to rent the room
• Rental box uses llAddToLandPassList() to add the user to the allowed list for the parcel
• Auto-return now leaves their items alone when clearing land
• User's rent expires
• Rental box uses llRemoveFromLandPassList() to remove the user
• Auto-return now returns all the user's items

This doesn't involve groups at all, but allows for a flexible land-management system using scripted items and settings already available in-world.

The allowed list could also optionally be used for adding extra capabilities for the build, object entry and script running options. ie: object creation would have three radio-options; everyone, allowed users, group. Also the ability to let allowed-users set the plot as their home location. But these are not as important as the ability to exempt them from auto-return.

Also, I would like to note that this feature would be independent of access controls. That is, scripts could add/remove from the access list regardless of whether the access restrictions are turned on. So long as "Exempt allowed list from auto-return" is turned on, then they would not be affected by auto-return.
This means it can be used without requiring the "ban lines" everybody hates, and thus can be used purely to keep land clean.