Since te sign is behind the sidewalk and some bushes, I doubt there are legal issues, but I'd look to see how hard it is to dig up or remove the sign. If the post is secured (i.e. in concrete), I wonder if you may be able to use those wholes for an advantage if you could get a cable or lock through them since it sounds like you could leave something attached semi-permanently.

If the sign was actually in a space, as others have said, I wouldn't park there but this is a different case.