This is an old thread. Have you accumulated more experience on how to avoid seizing in the chair?
I have been lucky until now. Previously during travel AS has never been a problem in my sacrum or hips. In fact I really never had any symptoms down there other than sore buttocks and thighs while sitting too long. My spine problems were only in my neck and between my shoulder blades.
But in the past six months I now have chronic burning, stiffness and pain sensitivity to movement in the sacrum and hips and I can feel damage accumulating. I will be traveling for about 30 hours with three connecting flights and the longest flight is 12 hours!!
I have had my sacrum painfully freeze for periods of a week a few times over two decades but I had no idea what it was. I would roll out of bed in the morning, lie on the floor and wiggle like a worm and eventually get my self standing to go to work (never called in sick!!) Hindsight informs me that the flareups were associated with gut dysfunction caused by NSAIDs, ironically! The first time I experienced the sacrum freezing I laughed so hard that it hurt like h*ll!
HLA-B27 neg; Mis-diagnosed with fibromyalgia in mid-20s.
Vague AS symptoms in 20s and early 30s (no diagnosis).
During age 47 (2015) from NSAID use developed complete axial inflammation (started from from skull and progressed down to sacrum). Included psoriasis. Trigger in scull was reverse blockage in a SCUBA mishap in 2013. Straw that broke the camels back was over use of NSAIDs. NSD works well but not perfect. Strong involvement between symptoms and gut integrity.