BruceS:I looked at your example site that you listed. From the gallery of photos for the site, I only saw 20-30 potential waymarks. There was at least one Texas historical marker, a few different Signs of History, there are also arches, trailheads, hiking bridges, I assume there probably is a Civil War memorial or two somewhere there. Unless there is another Reunion grounds near Mexia the site itself is listed on the National Register of Historic Places, thus it appears it would not fall into one cracks your proposed category want to provide for and would not qualify to be listed in it.
You are correct. There is no place specific to put the location itself.
Let us look at cemeteries. They have historical markers, statues, famous people, etc. Yet we still have a category for cemeteries.
This is along the same line. One site that can envelope the purpose of the site itself.