This is a more specific flag, but it has a huge use for specific servers.

Whenever a user tries to cross a regions border INTO or OUT of the region, Worldgaurd checks their inventory
If its empty, and the flag is on, the user is allowed in.
If it isnt empty, and the flag is on, the user cannot cross or teleport in.

If you could even add specific support for blocking if a certain item is in inventory, then that would be heavenly.
But the basic need is if empty or not empty. (Including Sidebar)

The use for my specific server is to prevent people from leaving an area that has no stealing rules, going to a rules free area, raiding a house, then putting the items back into the no stealing area, where it can never be caught or removed.