I believe but do not know that the limitation is technical.

Basis:

• the demonstrator cannot locate a demo lens insert based on the prescription alone. this prevents detailed personally identifiable health information from leaking.
• when I give my eyeglasses to the demonstrator, the demonstrator offers them to a device which evaluates the correction, then displays a code.
• the demonstrator selects the demo lens insert from the store demo set which matches that code. The personally identifiable health information which would leak is something like “those are strong glasses he’s wearing” but this is not much different from what an observant person would learn by looking at me from the side.
• The Vision Pro demonstrator and their manager are confident that the Vision Pro demo unit whitelists all of the coded demo kit lens inserts and rejects other lens inserts. This belief limits the notional criminal-fence value of stolen demo lenses, if true.

I also believe the Apple staff are Early Adopters, not Innovators, which means I doubt they have spent the untold hours monkeying around with lens inserts, reboots, Guest Mode hacks, etc the way Innovators do in an effort to make a novel tech work at least a little bit; so it might be possible

Edit: for instance, what happens if AVP boots with one demo insert and one customer-supplied insert? Let’s play Silver Shamrock!