Hi, so I can see the frame can rotate vertically so she can be in upright position. but how can it actually rotate around like a barrel roll? Most of those original turning frames would turn in a barrel roll fashion. They would rotate in the axis of the metal ring hoops by roller wheels that were mounted on the fixed part of the frame. You could incorporate roller 4 wheels on your frame as well to interface with the metal rings. This would give your frame total rotation control.
Also, it is unlikely she would be wearing a diaper when in such a cast. It is more likely she would be catheterized or use bed pans and most likely just wear some kind of pantyhose/towel to keep her covered up. Alternatively, and for dramatic affect to her treatment, she could simply be hooked up to urinary and rectal catheters 24/7 to remove waste to bags or containers. They have special 1" diameter large bore rectal catheters/speculums that can be connected to a colon hydro therapy machine to assist in irrigating/cleaning the bowels. It is common during long term bodycast immobilization to experience loss of regular bowel activity. So colonic irrigation machines are used in hospitals for immobilized patients.
It is also likely she would need oxygen support or a ventilator due to lack of breathing capacity while immobilized. Blood pressure monitoring cuff would also be required. Basically, put her on life support but keep her aware of her surroundings and keep her exposed and vulnerable and in awkward situations. A fire alarm and having to be wheeled out onto a public sidewalk while in a halo bodycast metal frame is always fun for getting embarrassed.