I may have solved my back problems today. Before I screwed my knee and lost 2 weeks of practice, I was getting frequent back pain from sitting in the same position for so many hours. I assumed it was my posture, but I couldn't find the source; I sit up straight in a relaxed, balanced position with minimal tension. I wound up switching positions (guitar on left leg vs. right leg) or standing every few minutes in order to manage the discomfort.
I got an info packet from LAMA on practice techniques today. There's not much I haven't already read on the topic, but I took note of their mention that a good posture shouldn't inhibit breathing. I used to swim avidly, and my lungs still seem abnormally efficient; I only require shallow, infrequent breaths while inactive. So I focused on drawing deep breaths while playing, and voila! Instant comfort. Time will tell whether this was a fluke, but I was already getting those old pains in my back in the last few days, and I saw a marked improvement today.