I do use a saddle chair in the studio. That said, I don't think the lack of a back will help your legs. This one that I have has a ring that is for your feet, but it's narrow enough that it might be hard on your knees if you use it. My chiropractor warned me about that one but as a motorcyclist - it doesn't bother me much, same basic position.
I do find that with the saddle chair it's more adjustable than a lot of the other adjustable chairs I've used, and I can use it at the frame, the cutting table (built to standing height), the kitchen counter and the sewing table (also at standing height)
For me, the adjustable height makes a big difference on my legs because I need different heights to quilt at depending on if it's micro fill or an all over type design. It seems to relax a little pressure by changing it up from time to time for me anyway.
The other thing that might help, and I know the saddle chair I have has it, maybe other chairs do too is the ability to tilt it quite a lot. More than my "deluxe" office chair. Tilting it forward may relieve some pressure on your legs, but probably only if used in conjunction with some sort of riser.