I don't know anything about this condition, but I wonder if Sally Swift's Centered Riding visualizations would be especially good for you. The book and video are easy to find, and worthwhile no matter what kind of riding you want to do.

I am totally not familiar with this condition either, but what about your torso position?  If you can keep your torso centered and straight, you will go along way to having a correct position by just keeping your arms and legs relaxed and letting them fall naturally beneath you.

Perhaps if you explain in a little more detail what your limitations are, we can give you some better exercises or visualizations to help you stay balanced. 

 Thanks for the info, that makes a lot more sense.  And I am going to guess that the absolute best thing for you to do is to ride with mirrors -- ideally one on each long side and two on each short side in the corners.  That way, you can SEE where your body is without having to move your body to look.  By first being able to see, you will then be able to place your body where it needs to be.  By first being in the correct position, I think this will help you develop correct muscle memory, which will in turn lead to you being able to have a better instinctual position on a horse even though you can't exactly tell by feel.