Corner crossing seems like something that could be solved by saying "where two corners meet there must be an easement xx meters wide to allow the public to cross". I can't imagine any reason why the private landowner should be allowed to put up fencing and signs that block the corner.

In cases where the only way to access a parcel of public land is through private land then an easement should be created. If a road or trail can't be built for whatever reason then a sign with an arrow should be the rule.