This sounds like it would be a good exercise to get the horse relaxed and pliable. I think that difficulties with this would be loss of forward motion and keeping the haunch from swinging out of the circle. Maybe over flexing at the neck if not enough outside rein is used effectively. Sound plausible?
All of the above and the correct crossing over with each step. Aligning the fore and rear so they stay straight but flexible. It is the ultimate in control and I can almost guarantee that even if you feel your turn on the forehand is perfect you will find every flaw and weakness when attempting this.
What I am doing with these threads is not to teach you how to do something but to understand what you are doing so you CAN think out of the box. Just because you can do one thing does not mean you move on and forget it and done now on to something else. Even the most lowly movements can be taken to the next step.
I am hoping questions are asked so when you go into a test and things do not go the way they are supposed to, ( happens ALL the time) you can at least minimize the error on the spot.