I just wanted to tell you about a solution of my own that you might consider adding to your site - Having found that a director's chair, as recommended by Kay, is quite comfortable because of the gap at the back for my tail bone to hang over, I recently thought that the same principle might apply with a collapsible wheelchair - i.e. the back upholstery is generally "slung" and and so are the seats, just like a director's chair, but they're more comfortable for longer sitting.
I've bought myself a wheelchair for use to sit at my computer - it's a Sunshine Medical (Tel UK 01384 446688,) "Breezy SL" self propel lightweight wheelchair that has a fully adjustable back rest (an "optima" tension adjustable back) which has velcro straps under the uphostery. The bottom two straps of the back rest can be loosened to allow you to hang you tail bone over the back without having to cut off any material from the seat. I just use a thin fleece on the seat to give my legs a little padding which also softens the seat edge - Since I got my wheelchair I've experience absolutely no tail bone pain whilst I'm in it and I've tried it out for hours on end.
My wheelchair was quite expensive because I particularly wanted a lightweight one, but I did try out some other cheaper ones that were equally as comfortable, even without the adjustable back rest upholstery, because they had quite a gap for my tail bone to hang over anyway. It's just a question of trying different makes out in a showroom and sitting for a while to test them out.
Hope this might help others - particularly in the work place - being it's a wheelchair you can just wheel it along to other rooms, meetings etc with ease. Could even take it to cinemas and theatres, pubs, evening classes etc etc.
I do get some strange looks when I push my wheelchair and then sit in it, but I couldn't/wouldn't do without it now despite feeling very self conscious at first. Occasionally, if I'm going somewhere with my husband I don't let on I can get out of it - he pushes me into restaurants etc and it saves me having to explain anything to anyone.
I would like to add that not only do I have coccyx pain, I also suffer with Fibromyalgia Syndrome (a chronic condition of unknown origin, although there are plenty of theories, that involves widespread muscle pain, stiffness, severe muscle spasms and overwhelming fatigue amongst other symptoms which include plantar fasciitis) - I'm always pleased to be in contact with others to share coping strategies/treatments.