Bukkit

A chest that requires no keys. Each player defined in a virtual inventory can access the chest.

You can add players by clicking on the lock.

It opens an virtual inventory with a close button(Wool Block with the name Close) in the corner.

Each slot represents a player, so to add a player to the lock insert an item with the players name.

To remove the player remove the block from the inventory.

Owner can add more players using a predefined item i.e. name tag with the playername on it.
Or just any item that has been named the player they want too add.

The players just name the items at an anvil, the default renaming items it costs exp. too so it kinda balances it to add players.

Maybe on first lock creation add owner to the inventory as default item to decrease the minimum exp. amount if you want to create a lock.

I can see this also having expansion possibilities with permission groups/factions/towns etc.
Each block type checks different things:
stick/cobble/dirt/wood etc = default player name
gold ingot = faction group
diamond = permission group