Busting out a shaving cream pie to make a point is downright unwieldy, but Instructables has plans up for a toy pneumatic gun that can shoot cartridges filled with tar and feathers. Now that just makes good sense.
The Portable Pneumatic Variable Payload Delivery Device is made of cheap materials (PVC pipe, a lot of tape, etc.) and is great for close-range goo blasting. The truly trigger-happy can even fill a cartridge with flour to make cleanup virtually impossible.
Now, this device might seem a little conspicuous for a town hall meeting, but any angry mob will tell you that nothing stimulates productive political discourse like a little burning molasses to the face. [PenfoldPlant's Instructables via Dvice]