You stated:
Sorry, that is currently not possible. I'd like to know what is involved that keeping the key is so important. If you could email me at [email]kelly@lindenlab.com[/email], I'd appreciate it.
My first guess would be llEmail. There is at least 1 resident run service I have seen that offers a 'name service' for objects in SL, so that their key doesn't matter (the object registers with the service on rez, and emails are forwarded to that object until a new registration is made I believe).
We may at some point offer a similar registry / presence process or service, but I don't know of any details or timeline for that kind of a feature.
This is actually mistaken. You can move an object from the Mainland to a Private Island Sim using Teleport.
The process involved is as follows.
1. Right Click on item setting out "in-World"
2. Select "Attach" from the pie menu (may have to select "more" first
3. Attach someplace on your body
4. Teleport to the Sim you wish to move the item to
5. Find your location
6. Right Click on the item and select "DROP" and you will drop the item to the ground
NOTE: Do NOT select detach as it will put the item into your inventory.
As of version 1.9 this process has worked perfectly due to the change in teleports so they no longer reset the item key on teleport.