OLDNSLOW: I assume from your post that it did not take 5 years of torso twisting to finally reduce your asymmetry, but rather that you started doing torso twists only in the past few months. Is that correct?
MOOGE: Not being your surgeon or physical therapist, I do not know all your particulars. As I understand it, though, cutting the intercostal nerves has likely left your lateral abdominal muscles -- the muscles around the bulge -- just flaccid and stretched, with no innervation to even keep their muscle tone.
I do know that the human body's ability to compensate to repair and rehabilitate, and even (more interestingly) to reprogram itself, is sometimes amazing. Several stories by the neurologist Oliver Sacks in his books show that even the nervous system can reprogram itself. In this situation, even if the intercostal nerves do not grow back, and (if they do grown back, a process of months) even before they re-innervate the muscles by growing back, proper exercise may "reprogram" those muscles to regain their muscle tone and reduce the flank bulge considerably.
If all that is your situation, sit-ups ("stomach curls") may help a bit, but they strengthen primarily the abdominal in the front -- producing the "6-pack abs" of body builders. The torso twist is better because it involves those lateral abdominal muscles, and thus will involve those muscles around the flank bulge that currently are just flaccid & stretched. Probably doing both exercises would be best. After the surgeon & PT say it is OK, start with light weights or resistance for both the curls (sit-ups) and torso twist. If you want to try this, PT should be able to give you a schedule of starting light and progressively increasing resistance or weight for both exercises. PT should be able to tell you as well how to improvise to do the torso twist without a fancy, expensive, machine.
PT may also have suggestions for additional exercises.
BTW, if you do elect to start these exercises when given the go ahead, and if your get your flank bulge to resolve, that may be worthy of your case being written up as a case report in the American Journal of Transplantation [or similar]. Although flank bulge may appear to be not a frequent side-effect of open nephrectomy, it obviously does happen. I am not aware of a journal article that discusses how best to resolve it non-surgically. (You could ask PT to check the PT literature as well.)
SO, I suggest that, before starting to exercise, you ask someone to take several high-quality "before" pictures of you, with a good digital camera with high pixels. Take several views (front, back, front off to the side, back off to the side, etc.), focused on your right side from your lower rib cage to the top of your hip bone. The pictures would be both for your own sake, but also in case your experience should be reported in the surgical and PT literature, to help those future living kidney donors receiving an open nephrectomy who end up with the same condition.
AND, in any case, I & we LDOers hope you get back to normal [= "normal-with-a-scar"].
Bill