I've never heard of priority seating on the monorail but i've never stayed in one of the DLR hotels so i could just be out of the loop.
But here's some info on the monorail. Anyone can ride it who has a valid ticket into DL. If you get on at the DTD station you have to get off at the TL station but if you get on in TL you can stay and make a round trip. Also, in my experience, it takes SO much longer taking the monorail in than it does to just walk. I keep trying it thinking it's not as bad as i remembered but it is. The last time i tried it was in late february. I thought it would be fairly fast since it was the middle of the week and during "off season". But the problem was that because there were less people, they only had one monorail going so it took a long time for our turn.
My advice would be to ride the monorail as a ride (or for the novelty of it) but don't ride it for convenience or speed.