I had the next best thing once about 3 years ago. I covered my sign with WD-40 and grease. Some little leftist snot came by while I was actually outside around the side of the building. I saw him steal the sign and throw it in the back seat of his car. He realized what I had done to it as he was pulling away and I heard a string of profanities all the way down the street.
Aren't medical maggots available now?
How about the really thin sandwich bags maybe painted or some such or with the maggots in a few layers of wet towel or some such . . .
The symbolism would be worth it . . . as would lots of maggots all over the back of a car . . . especially if they have little girls.
One could put wasp nests inside but that would be too dangerous for those with allergies. Scratch that.
Wouldn't know how to collect a critical mass of gnats.
Various kinds of worms could be interesting.
Then there's powdered sugar. Dangerous to inhale, though.
Perhaps some powdered sugar mixed with flour. Gets difficult to clean up and easily sticky in humid climates.
Seems to me the bags would do well to rupture in key ways immediately when tampered with but not really leak out visibly for about 2-3 minutes. I suppose cutting some slits every inch or so in the sides of ziplocks . . . and putting 3 or so layers of slit bags around one designed to rupture easily but without slits--that might work.
Or perhaps a thin fish line buried inline with the stake . . . with some barb or nail or some such which would rupture the inner bag easily without being too easy to detect . . .
I'm getting too much of a kick out of this. I should stop!