Why do you need anti-theft measures for a folder? I thought the idea with folders was you're able to carry the bike with you most everywhere, and not leave it locked on the street?

That said, I remember someone suggesting a cable or chain for standard bike, but run inside the saddle post and saddle tube. I don't know how you can fasten it to the frame inside the tube (BB? Bottle cage eyelets? Something else?) and whether that would interfere with the folding mechanism.

--J