This is not cool at all. Because then the rest of the general public simply views the spot as "no trespassing" (even though it's not really that status), and you are simply seen as someone up to no good if you subsequently go in later.
There was a fellow in a town near me that did EXACTLY what you're saying, since he was a sign-maker, by profession. He got in to a good old-town demolition site, where he was digging bottles, and didn't want anyone else to go in there, when he didn't happen to be there. So he made up his own sign with ..... d/t his profession ...... he could make very "official" looking (as if the city had placed the sign there), saying something to the effect of "no bottle digging, no metal detecting, no trespassing, violators will be prosecuted" blah blah blah. Well the newspaper somehow picked up on this, and took a photo of it, and put it in the city's newspaper (since, I guess, they were running a piece on the urban renewal project going on in old town district). You can bet it simply gave a "black-eye" to metal detecting, etc... And the sign-maker fellow got away with the ruse, because he had a brother or nephew or something on city council, who gave him a wink wink to do it.
Some md'rs saw this in the newspaper, and had the ominous feeling of "oh no, laws and rules coming down direct from city, specifically aimed at us, the sky is falling, we're doomed, we need to form solidarities, blah blah blah". Later the rest of the story came out (to those of us who dug a little deeper into the origin of "the sign"), and you can bet we did not take kindly to it at all! The damage was already done in the md'ing community.
So, no, I don't think it's a good idea. Not even if it is generic "no trespassing", it simply kicks a hornet's nest, which you don't need to do, in a hobby where this (signs, skittish hunters, etc...) is already an issue we deal with all the time.
if someone did that.
