I dunno about the legality of it, but I've noticed that when the city puts up signs, or paints a mural, there's usually something that has the mayor's name, and "City of Boston" in the corner.
Now, forging your mayor's name might not be the best idea, but having an official stamp/seal that said:
Charles Fortinary
Director of Public Spaces
Permit number #87362346
Might lend a faux authority to the gasm.