OK my theory, a theory which is mine and belonging to me....
The shadow of the boom is on the centre line of the Buff and they are in at least a 70 AOB manoeuvre, meaning the sun must be pretty low close to the horizon. To me the terrain and horizon in the photo do not look like they are being illuminated by a low sun.
As to other considerations:
If this were a routine element in IP training course syllabi we would have seen more pics like this.
If I were taking the photo I would have dozens of them, come to think of it I would go for video.
The USAF training world is risk averse, I do not believe there is a training benefit to be gained from manoeuvring two types in such an extreme manner.
Both types are very old and must be worried about structural fatigue, again why waste fatigue with little or no training benefit.
KC135 (and KC10) have a very satisfactory set of visual markings for position capture and maintenance. They also have director lights on the bottom fuselage which the boomer can operate to guide the receiver aircraft.
The photo is taken from a point over the right shoulder of the boomer who lies in a prone position. The boomer may be strapped in (but I can’t recall ever seeing straps) and for sure there are highly unlikely to be straps for such a circumstance.
Lastly, the wings of the Buff don’t look very bent to be under any g (granted the manoeuvre may be unloaded or ballistic at the time).
I stand by to be proved wrong.
Tarnished
PS
Found this pic:
Looks like there is a strake across the window as well, and the bed/bench on the right of the boomer looks like it would be difficult to stay on it and take a photo in such a manoeuvre.