I have never seen one done but that sounds like my kind of pleasure class. I love horses who are in perfect control but actually have some energy in their step and their head at or slightly above level (where it would be in a natural setting). If you get it done, I would love some videos.
If I was judging something like that, I would look for:
Control; the horse should be willing to travel at a consistent speed on a loose rein or with only minimal checking.
Comfort; the horse should appear comfortable to ride. Nobody wants a horse that is going to leave bruises on your thighs every time you trot.
Responsiveness; cues should be minimal, not much more than a nudge with either leg or rein.
Levelness (is that even a word??); the horse should have a level topline but slightly above or below is acceptable. Undesirable headsets are peanut roller and head-in-lap. However they carry it, it must appear natural.
You could always have a rail class that is like a true WP class and if there is enough interest, also have a class with patterns like they have in trail or horsemanship classes.