What I would do instead is take one of those dye packs that they hand bank robbers, and integrate it into the seat. Fix it so that it fires some number of seconds after someone sits down on the seat. Add in a proximity switch of some sort (kind of like the immobilizer keyfob for a car), so that if you have your little keyfob with you, then you will be able to ride the bike without the thing firing off.

See some kid walking around with a bright orange crotch, and you know what he was up to.