Pain in the thoracic part of the spine (upper back pain) is especially challenging since it is where all the ribs are attached and it is not as mobile as the lower lumbar spine or the upper cervical spine.
Inversion is still the best answer as it provides progressive decompression. Each joint being decompressed by exactly the same weight that it normally has to hold when upright, so in this sense, inversion works as well for the upper back as any other part of the spine. However, because it is not as mobile, rotation and side bends do not affect it as much as the lumbar.
To help move the thoracic joints, I augment inversion with back extensions on an exercise ball. Another option is for a helper to pull your hips forward when in full inversion while you grip the bed with your hands over your head. This gives a back extension while inverted which is very effective; this is a very advanced technique and should not be done until fully comfortable with full inversion.