The High Point XC ski center trialed allowing fat bikes a few years ago. I think it was just allowed on the snow shoe trail. I don't see anything on their site about it anymore, so I assume it didn't go well.
I called the high point xc ski center yesterday and asked the same question. They allow fatbikes on the snowshoe trail only. The monument trail is just under five miles. When I asked about conditions she told me they had 4” of new snow on top of a 4” base.