Wow, I didn't realize that this would become as big a topic as it has... though I'm glad it could become a subject of debate. I am very grateful for the advice and knowledge as well. To be clear, I never intended to imply that western riding does not involve collection or asking the horse to use its hind quarters. I am very aware that the opposite is true. I guess you could say I'm just a little gunshy given Kasja's history about how she will respond to the lower headset.
Before we got her, she was trained very much in the "rolkur" method. Because of this, she has the tendency to lean into the rider's hand. If you aren't constantly correcting her and asking her to support herself and lift her forehand, you'll literally feel as though you as the rider are attempting to support the entire head neck and shoulders of a 1200 lb horse. This is what I meant with the "fighting gravity" comment. The consequences of this include her becoming lame because the extra weight and strain aggravates the arthritis in her front pastern, she gets spookier because she feels off balance, and she begins to stumble over herself.
Thanks for the input tinyliny. Kasja has advanced enough in her dressage career that we have ridden her in a double bridle, so she is somewhat familiar with the curb action of the weymouth and she does not seem to have too much difficulty with it. I will likely begin this western riding style "testing phase" by riding her in the double bridle a few times and working on using less snaffle and more curb rein. She is very skilled at following the rider's weight and leg cues when it comes to turning and bending, so the neck reining and such should come relatively easily.
The different type of contact is what I'm questioning when it comes to Kasja remaining balanced and collected. With the curb style bit, she will not be able to lean into the rider's hand as much as she has been while riding in a snaffle. But does this mean that she will begin to carry herself a little more (which would be ideal)... or will she "nose dive" without the rider's constant reminders to pick herself up?
I guess there's really only one way to find out: I'll have to give it a shot.