You could try it but i think after a while regular visitors will realise it is fake and take no notice of it.
I took a couple of photos from a supermarket car park one evening of the aerials on a mast above a police station, the reason i was interested was they had put a full sized plastic Eagle Owl on it to scare off the real birds, who were all lining up neatly on the aerials to roost.
I usually find that the 'Black' bird poo is from Wood Pigeons which have been eating Elderberries. They seem to love these and I have seen them sitting in the trees inspecting the berries waiting for them to ripen, then they gorge themselves. I have seen them hanging upside down just to reach a bunch that is out of reach!
My main problem is gulls which seem to use my car for target practise, and very good they are too! My neighbours car parked only a few yards away seems to escape their attentions. Could it be the colour? My car is silver, my neighbours' is white.