It's possible but I wouldn't want to sit on anything like that for fear of a malfunction. Maybe a decoy seat though. After the ride remove your seat and replace it with the decoy seat. As for mechanism, I favor something along the lines of a bang stick used by scuba divers to protect themselves from sharks. Sitting on the decoy pushed a primer down on a firing pin creating hot expanding gases that push the spike up in the offender.
Makes a load pop along with the pain.
You're lucky you caught me in an evil mood.