I think as long as you aren't pulling the horse into a frame, but simply holding the bridle out in front and encouraging the horse forward with your seat and legs, it shouldn't be uncomfortable.
Now I don't do high level dressage, I barely do any dressage as my horse is green and we're just trying to get him comfortable, but there was an occasion where I worked him in a rope halter (which is a little more bitey than I suspect a bitless bridle would be) and he kept himself nicely together without bracing or showing any signs of being uncomfortable.
It would probably take some effort but I don't think it would be impossible or painful for the horse.
Just one girl's thoughts though :P
"Strength is the ability to use a muscle without tension"
Last edited by Skyseternalangel; 05-29-2012 at 08:23 AM.