Dave,
The Feldenkrais Pelvic Clock is a lot like the Pilates Clock or Bowl. We just don’t turn our heads! 
Pilates Pelvic Bowl/Clock: Lie on your back with your knees up and legs and feet parallel and arms by your sides. 12 at the pubic bone, 6 at the belly button, 3 at the right hip, and 9 at the left hip. But imagine that you have bowl created by the pelvis and there’s a marble rolling around in it.
Start by tucking your pelvis so that your back flattens/presses into the floor. This rolls the marble to 6 on the clock face. Pick up your left hip slightly (no more than 1 inch). This rolls the marble to 3 on the clock face. Now lower your left hip and arch your lower back. This rolls the marble to 12. Now lift the right hip up about an inch. This rolls the marble to 9. Lower the right hip and tuck the pelvis. The marble has now returned to 6. Continue for a few revolutions and make it as smooth as possible.
Change sides/directions and repeat.
Another hip loosening exercise is knee sways, which can be done a couple of different ways.
Knee sways: Lie on your back with your knees up and thighs close together and your arms either by your sides or out to make a “T” shape with your body.
One way is to let your knees float toward the floor on one side (say the right) keeping both feet touching the floor. Your thighs will slide past each other and your feet will roll to the right sides. Let your left hip come off the floor, but make sure your left shoulder stays firmly attached to the floor. Scoop your abs and get ahold of the pelvic floor and bring your knees back up. Try it without using the pelvic floor and notice the difference. Repeat 4-5 times, then change sides or alternate between sides.
Another version is to keep the thighs tightly squeezed together and your left foot will come off the floor when going right. Be sure to scoop your abs and use the pelvic floor to come up.
Anothr version is to use the whole exercise as a way to stretch. Just lie there with your opposite shoulder firmly pressed to the floor while trying to get your knees to the floor on the other side. This stretches the back, pelvis and left side (if the knees are going right).
OP - You report that you cannot lift your right leg off your horse to kick him, is that right? Can you stand up while on the ground and abduct or bring your right leg away from your standing leg? Do you have to hold onto something to do this? Is it harder to abduct the right leg than the left leg?
Watch yourself in a mirror or have someone spot you - what do you do to compensate for your right leg weakness? Do lean one way or another? Without holding onto something, what do you do with your arms? Your torso? Hips, knees, ankles, head?
Get on your horse and have someone spot you from behind, in front, and from each side while you try to abduct your leg off your horse. What do you do with the rest of your body when you use your right leg? Left leg? I want to know about arms, shoulders, head, neck, torso, seat, opposite leg (if possible).
Are you centered in the saddle? Do you have even weight in both seat bones? Are your legs the same length from side to side even if the leathers are absolutely even? Are your legs in the same position from side to side? At a halt? Walk? Trot?
At a trot, which circle is easier to do - going right or going left? Which one falls in? Which one drifts out?
Look in a full-length mirror without as few clothes as possible and draw an imaginary plumb line down through the middle of your nose. Is your head on straight? Does your belly button line up with the center of your collar bones? Are your hips level? Knees? Ankles? Shoulders? Is there the same amount of space between your waist and the elbows on both sides? Do you prefer to stand on one leg? Which one?
Do you stand with your knees locked? How much curvature to your lower back? Upper back? Where’s the weight in your feet - inside, outside, toes, or heels? Are your feet the same? Do you stand so that a plumb line goes through your ear, your shoulder joint, your hip joint, just behind your knee joint and just in front of your ankle joint?
Have you ever had any injury of any kind? What happened? What was injured?
It might be that you don’t need to loosen the joint so much as you need to strenghten it and align and strengthen the rest of the body as well.
All of these things matter.