Well, since Dad would like to see some work applicable to the already stitched version of the image and no one else has stepped forward, here's a bit of work I did on it ...
Like iDad's suggestion, I also used a layer in the "divide" blending mode, but for my denominator, I used an estimate of the local brightness in the foreground. I got this estimate by selecting the foreground, then using extreme motion blur in the vertical direction to vertically average each vertical slice as you go across the image. I then blurred it a bit isotropically (to hide the fine vertical lines), used some blendIF settings, and after a bit of tweaking, got what you see below.
Also, since the original was so dark and flat, I brightened it up considerably and applied a good bit of local contrast enhancement.
A lot of the original banding is still quite visible, but at least it's on a smaller length scale, it's not so dark, and generally, it's not quite as visually intrusive as the original, at least IMHO. Topping my version off with purely manual methods (eg, dodging and burning), would easily improve this.
Attached below are:
a) the original - downsized to display in line for easy comparison;
b) the estimate of local brightness in each vertical strip;
c) my quick and dirty tweaked version.
That being said, if, by some chance, the OP has the unstitched component images still available, he is essentially guaranteed to get better results using PanoTools to re-stitch the image instead of the above sort of after-the-fact desperation approach like I demonstrated.
HTH,
Tom
PS - @iDad - I would love to see how the method you proposed worked. Maybe if one is going to do everything manually (like you suggested), it doesn't make sense to bother going through the estimate of the local brightness like I did.