Not what you're looking for, but would this do instead?
http://store.velo-orange.com/index.p...t-skewers.html
Basically it's a normal allen wrench hole except there's a post in the center so you need a hollow allen key to undo it. Sure you can buy them, but most thieves probably don't just carry them around.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"