While I understand the intent, and have done similar with previous vehicle (SUVs with a rear sprayer are nice for this), please keep in mind all liabilty issues. Should any damage be caused by this, they're right behind you with a nice clear view of your plate.
This wouldn't be as easy to get out of as the MN coyurts decided speeding and/or stop light tickets were. "It wasn't me driving" wont get you out of any tickets or litigation based on an intentional mod.
Your best bet would be something that does not cause permanent damage. Water, soap, stuff like that, would be as far as I would go. A light mist and a dusty windshield should be enough to get them to back off.
A good place for the sprayer might be in or under the rear spoiler. Painted to match the body, and it would be virtually invisible....
1998 SE Midnight Blue Pearl
Gutted airbox, FRAM AirHog, removed MAF Screen, eBay clear corners, extended brake lights
I'm so much cooler online