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, vague AS symptoms in 20s and early 30s
1993:fibromyalgia (age 25)
2013.07:Scratched by a sick cat
2013.08:Reverse blockage in a SCUBA accident
2013.09:Strange sore throat then meningitis
2014:Chronic inflammation at the base of the skull
2014 to early 2015:excess NSAID use developed complete axial inflammation, included psoriasis
NSD helped well and but was not perfect
2018.07: weak +'ve tests for borrelia, babesia, bartonella and mycoplasma pneumonia using Armin Lab, ANA=equivocal