Have you tried using a balance ball as a chair instead? I don't know how practical that would be for sewing but I know many people who've gotten great relief that way.
I have pain in my lower back after sitting too long at my sewing machine. I use heat therapy patches that I get at teh Dollar Tree - they're just mentholatem (sp?) on a patch but they do make it feel better.
I've seen pictures where women have propped up their machines with door stops to help avoid hunched shoulders. However, the pictures always show them putting the stops in the back so that machine tilts toward them. That would make me hunch more. I'd rather have it tilt away from me so that I can more easily see the sewing area.