Andrew, let's just cut-to-the-chase here. Even if there WAS some such restriction (if you asked enough lawyers and archies, high enough up chains-of-bureaucracy in some fed office across the continent), ..... seriously now, who has entered into such a contract or restriction ? Is it you? No. It's the homeowner. Any such restriction is for HIM to know and fulfill or believe in, or do, etc... So if the homeowners says "yes" (as you say, you're getting permission), then that's between him and whatever governing agency he thinks holds say-so over his property. You can not get into any trouble. Notice I'm not answering the question from the point-of-view of someone in Washington DC passing out those plaques. Because I have no doubt if you asked that same person if you can detect a sandbox somewhere, he'd probably also say "no". You can ALWAYS find someone to tell you "no" to any place you can think of. The point here (IMHO), is who's responsible for the place. And I say that's the owner. If he believes he has the right to grant permission, then why oh why oh why does anyone argue with a "yes" ? ha I rented in a house with a city-issued historic plaque on the front door (all the houses in our multi-block area were the historic district of our town). And do you think for one second that I didn't metal detect my own yard ?
