private cancellations in USA

While I am familiar with private overprints (usually for propaganda purposes such as support of the Italian Social Republic, National Socialism and so forth) not only elsewhere but in the U.S. (particularly noteworthy being the "Pray For War" used by a woman during the Vietnam conflict and the mysterious private overprint on the Florida statehood stamp), I was wondering if anyone had any information on private cancellations on U.S. postage under the Private Express Statutes and outside their occasional use on U.S. personalised postage (technically defined as meter labels). This would be a fertile field for exploitation by creative and philatelically-minded individuals.