I had a lower back injury and was on my back for 3.5 months. Two herniated disks and narrowing of the nerve channels and I was in severe pain and could not ride at all. Before this happened I was building a bike for long distance cycling and so made a tweak or two to make it a "comfort bike". added a body float suspension seat post to absorb shock to my low back. Set up trekking bars with a fairly upright option and the frame I used (VO Pass Hunter) has a nice ride with the 32 mm tires at 80 rear and 60 front.
The back is getting better and I rode it 40 miles yesterday with no ill effects to my back. I am 70 so need a little comfort anyway.