Hi. I found your site while searching for relief from a sore tailbone that had lasted for weeks and was impairing my ability to sit and perform my work at the computer.
Recently, while searching for a new chair, I came upon the idea to use my neglected physio-ball that sat in the corner of the room. Almost instant relief. It's been two days and I've been able to sit for over eight hours over with no discomfort. I use a small ball that's pretty soft.
Just an idea. I hate chairs, they cause so many problems.
See Gym Ball on Comfortable chairs