I know this won't make everyone happy, but why not have two buttons to press on the item? Or perhaps two versions of the rod: a minor and a major?

One minor activation, which makes it immovable to a "scene", whether that be the ground, a carriage, or an airship. Functioning as door stopper or climbing hand-hold but no damage to the moving vehicle as it becomes a "part" of the vehicle.

The other is a major activation, which makes it immovable relative to the plane. So it would not harm stationary objects, but could cause some chaos to moving objects. In the above example regarding being set off in the gears of Mechanus, I would imagine such a "plane-relative-set"-rod could cause quite the headache, to say the least. I could even see such a magic item as being completely forbidden and any Modron or any other citizen of Mechanus to respond quite forcefully to any party who would bring such a potentially chaotic item to their plane.

However, I could see "plane-set"-rods being very well recieved in the the Outer Planes the Abyss, Limbo, and Pandemonium.