I just had this totally weird idea, guitar hangers that have a lock on the front of them, in the style of those U-shaped bike locks. The 'U' portion that holds the guitar would be too small for the headstock to be pulled through, so your guitar could not be stolen (in theory).
Here is a small version of one of those bike locks. I don't know exactly how big this one is, but you could perhaps weld a bolt onto the middle of the back of the 'U' so it could be fastened into the wall.
http://www.bikebarn.co.nz/zoom/bike-locks/ressettable-handway-u-lock
Perhaps it may be desirable in a store setting holding the more expensive instruments?
Here is a small version of one of those bike locks. I don't know exactly how big this one is, but you could perhaps weld a bolt onto the middle of the back of the 'U' so it could be fastened into the wall.
http://www.bikebarn.co.nz/zoom/bike-locks/ressettable-handway-u-lock
Perhaps it may be desirable in a store setting holding the more expensive instruments?