Another practical way to alleviate the pain might be to shift from a piano bench to a straight-backed chair for awhile. It must be of the correct height, of course. I saw Radu Lupu and Arturo Michelangeli both use chairs in the past, and assumed that they too had back problems at the time. If you try this, you'll need to experiment a little. With a bench or artist bench, we position it on the floor such that we sit forward on only the front half while playing. My sense is that you might have to sit a bit farther back in the chair, not leaning on the back, yet being close enough to do so comfortably when opportunities arise to gain some relief.
One other point: Upper back takes in the shoulders. Sometimes pianists, unknowingly, tend to raise their shoulders during deep concentration on their playing--a manifestation of tensing up. Because they are totally unaware of it, they make no attempt to lower the shoulders either. That starts pain in the neck that quickly refers into the shoulders. Tip: Consciously ask yourself, "Are my shoulders raised up high?" If you find that, you'll lower them immediately to the natural, relaxed position. If you catch yourself doing that, keep reminding yourself, and eventually you'll break the habit entirely, as the conscious becomes incorporated into the subconscious.
I hope this helps.
Topic: Back Troubles