I know there are companies that make large versions of pepper spray alarm systems. These fill the area with pepper spray when they're set off.
It shouldn't be hard to do so in a car(smaller volume and all). Might mean you have to air out and wipe the car down afterward, but nobody will be able to get away with your stuff.
Simple key interlocks should be relatively easy to set up. You could probably get away with a completely analog system... But that would require more power in the armed state. You might wire with pressure or completed circuit switches to prevent removal of the head unit, amps, and speaker grills(grills prevent you from seeing your speakers, but they also can prevent accidental damage or someone easily destroying them out of spite).
Mount the spray unit under the center of the roof, and run wiring under the head liner. When someone pops a door, a regular car alarm(take your pick of brand) goes off. When someone tries to rip your head unit, speakers, or speaker grills off... OC spray goes off.
