Grave wyrms

This range of products is made to be inserted into one's corpse if, and only if, one donates one's body to medicine. The devices are free, donated by a charity. The installation is done by volunteer morticians. The idea is that if this product range is available more people will consider donating to medicine instead of the standard pickle and plant. The possibilities include:

A snake-in-a-body: a rubber snake on a compressed spring hidden in an arbitrary place ready to strike medical students when they lift, say, the liver.
A robotic imp
A ghost consisting of some canned fog, and a recording from an electronic device.
A recording device designed to start playing as soon as knife hits skin, saying things like "Put that knife in me one more time and I'll shank your Nana, that's right she's down here with me"
Compressed air
A helium-filled bladder
And so on.