A forest ranger told us that trails that were prohibited would be indicated by purple paint or physically blocked off. Like I said, we were able to do the portion from Old Hwy 7 to Sand Gap. No problems with officials, authorities, upset land owners, etc. We used the MVU on their site as well as the paper ones in the ranger district. As for the other sections of the "High Water Mark Trail", I cannot speak to them as we didn't travel them. We also were not the only ones using the trails, plenty of SxS and 4wheelers. I feel that if we were ever somewhere we shouldn't have been, it would have been a field day for the authorities and we would have known about it.
I see you are located in Conway, so I would assume that you would know better than myself, but I know there are also a lot of 'rumors' of trails being closed, or marked in the area. I say rumors, because there are plenty of people claiming the complete opposite. The Kiamichi Trail comes to mind when bringing this up, yet I had a neighbor that traveled K-Trail three weeks ago with his family with no problems, marked or blocked trails. There is also an Overland group based out of the Ouachita and Ozark Mountains, who try to keep everyone in the loop on what is and what isn't, if you know what I mean.