I have been going through the same thing as the OP, so I do literally "feel your pain" - I had an epidural injection to alleviate the situation somewhat. Even US domestic style F seats have been quite uncomfortable.

Find out from your treating physician what sort of pillow/back support will be best for the particular condition you have. Also, you may want to consider some acupuncture for some temporary relief.

Query - once in the water, your scuba gear will be fine but aren't you going to have any issues with the gear prior to getting in the water? And getting out?