My horse is a pretty forward moving horse, so I don't think we could successfully do western dressage. Not to mention I'm not a "western" show person. I did see that you can show in a snaffle bit, though. That's nice. They're a lot cheaper than show curbs and I don't really want/need to use a curb with my 4 year old.
I will say that having my trainer encouraging it makes me think he might have some level of success. At least at lower levels. I might consider trying to find a way to take him in an intro level 1 class this spring. I just have to find a ride and whatnot.
It can't hurt just to give it a shot, right?