Seems like there are lot of aerosol concoctions that could be employed.
Whipped cream could be interesting.
"Yes, officer, he attacked me with whipped cream. Here is his license number."
Cheese whiz in a pressure can?
Isn't there a water based paint that comes in an aerosol can. Like they mark construction sites with.
Chemical fire extinguishers shoot powder. Just fit a 5th exhaust pipe for stealth reasons.
If you are naturally vindictive, there is the insulating expanding foam. That stuff is he77 to get off of almost everything. With some luck, the tailgater will use the windshield wipers.
I guess you could get into trouble with those paintball splat things.
I do understand the sentiment about just letting them pass by. But, that does reward and encourage further bad driving behavior. I think there is far too much bad behavior on the roadways today, and the police just don't seem to care about it. There is something to be said for society policing its own behavior. Police can't be everywhere where bad behavior exists.
I like the Gallagher routine where all drivers have those pistols that shoot suction cup darts. Then when the police see a vehicle with more than 5 darts stuck to it, they get a ticket for driving stupid.