If the subject distance is large, as is often the case with landscapes, parallax is not a significant problem. I would be interested in Bart's comments if he is still following this thread.
That's exactly correct, one can only correct for parallax after the fact
at 1 plane in the distance (either foreground, or background, not both at the same time). Since at a distance the parallax in pixels is smaller, due to magnification factor, we are often able to blend the distant features in a smart fashion, hard to immediately spot the errors by eye. Only by rotation through the NPP there will be no parallax, and thus no errors, at any distance
. Therefore, adding the rotating clamp on top is mandatory.
What is entirely possible though, is to tilt the camera's optical axis to a non-level angle (e.g. down the mountain), and do a single row stitch (with NPP rotation) at that angle. In the Pano Stitcher software, one then uses the same Pitch angle for the entire row, and get a perfect parallax free stitch, where only the excess space need to be cropped.
Attached is an example of that, where I shot 3 images at an angle of some 30 degrees down, but rotated the camera through the NPP (rotating clamp on top of the ballhead, with MPR-CL II in that clamp to align the NPP), so no parallax issues in neither foreground nor background. The first attachment is just the rough stitch centered in view.
The second attachment is the same stitch but after moving the horizon up by adding a Pitch angle to all three images to remove keystoning in the background (a leveling plate would not have been able to achieve such an angle), which 'levels the image' although it will introduce perspective distortion in the floor as if the shot was taken level with a huge image circle and wide angle shift.
And the third attachment is the result after cropping, tonemapping, and a bit of cloning to fill in some details.