After you park the bike (large gear, large cog) move your levers in the oppose direction. If anyone tries to "pedal off".... they'll drop the chain with their first stroke. Just remember to return the levers to normal before you ride off. The tip works with indexed and brifters as well.

But with extra nice and collector bikes.... I'd worry about workmen leaving the store seeing the bike and throwing in in their truck or van. Even if it's properly locked... they have the tools and they know how to use them.