Good thought. The bump to the mount we are about that gives a 5# horizontal force will most likely not be perp to the axis of the pier so let's take a look. Assume an 8" steel pipe pier 48" high and, instead of bumping the pier, we bump the end of the counterweight shaft. We'll assume that is 16" from the pier center line and give it a 5# bump. Angular deflection is 0.25 arc-sec. So, we have a rotation of 0.25 arc-sec AND a vertical deflection of something less than 0.167 arc-sec. Still very acceptable. If we get picky, the values in the table are minimum deflection with the 5# force acting perp to the pier.
For a fixed height pier, the easiest way to reduce rotation is likely by increasing pier diameter.
I wondered who would be concerned about torsion but then I saw your Zerxes mounts and see why you would be getting that detailed. Excellent work!
For peeps with less than your mount, we can live with the simplified deflections in the table. There are so many variables on a specific pier, like how the connections are detailed, where you hit it, at what angle, is it really a 5# force, rat cage or not, etc. that the specific math can get way beyond the margin of error for a backyard pier. The idea is that, if we design for minimum deflection and use the table to get under 0.5 arc-sec, other forces, like torsional rotation, won't be a problem.