I don't have any particular recommendations re: specific chairs but I wonder if one approach might be to think about how to not use any one chair/position for too long. There are quite a few options now for standing desks, there are "kneeling chairs," exercise balls or exercise ball chairs.
I would guess that if someone is in one position all day, there is no chair that is going to make that feel good for the body - the body is meant to move. This would of course be worse with AS. But, if you could rotate through a few options during the day with stretch/walk breaks built in, maybe that would help?
Of course if you have too many options then your office mates might give you crap about your office looking like a office furniture showroom so there's that to consider : )
Since I sit a lot for my job, I'm curious if others have any ideas though!