Your plan of two points or spud mount type pipe sounds good to me. A Cheseborough normally designed for 2" aluminum pipe but can at times as assumed do 1.7/8" Sch.40 1.5" steel pipe in hanging a light but I would say not persay prevent it from slipping either on axis of the mounting
bolt given a single clamp or in rotation given a friction coeficient by way of tension on the clamp verses serface area of the aluminum clamp verse the size and type of pipe clamped to.
Long story short, if a clamp designed for holding to 2" O.D. aluminum pipe is used to clamp to a 1.7/8" steel pipe in this way, no matter how much you dog down on that clamp in possibly causing it to fail, you cannot achieve sufficient surface area on the clamp to prevent it from moving should the weight of a pushed follow spot be on it in an usupported way. This much less various blots mounting
fixture to pipe if left able to swivel by way of tension could loosen as a possibility.
Can support the weight yes, best way to support the movement of the
fixture no.
I was intially thinking span two clamps by a piece of steel or aluminum which at its center the
fixture mounts to by way of fiber or teflon washer between
fixture yoke and plate. I better like your idea of hanging from both top and mid-rail as I think I understand it. Say some form of L-shaped steel which by cheseborough mounts to both rails and has on the L' the
fixture mount. Not going to move at that
point and more within the concept for the clamp for holding to the pipe.
This concept if even
Unistrut and
Unistrut hardware making it up with the clamps installed on it would I think work better.