I just worked on lateral flexion, vertical flexion, and some round pen work. And I did it all in a girdle! Maybe a better term is an orthopedic, orthotic, lumobosacral spine stabilizer, but it sure feels like a girdle.
After some excitement when I couldn't move fast enough to catch him quickly in a small paddock, we settled in for some work. He did great. I even had him at a flat walk for about 3 times around the round pen. Our best effort to date on a flat walk was maybe 20 feet. That means I'm better able to get him in, and maintain, all of his speeds: the flat walk, the corto, the largo, and the gallop. That took about 2 weeks and 8 different sessions. I'm going to keep building on this, and keep him in his various speeds for longer intervals. I feel confident this will translate into better speed control from the saddle.
Ultimately, I'd like to be able to trail ride him in a flat walk to keep pace with my QH buddies (let them keep up rather), and use the corto and largo as spine sparing speeds to keep up with the faster parts of the trail ride.