Pillars should be valid structures on top of retaining walls. It makes sense that you can't drive over them or build actual buildings on them, but what effectively amounts to scaffolding should be allowed.
Similarly, very short placement of belts/pipes should not require a pillar at all. This is currently the case for length 1 objects but not length 2. In the attached image, the belt should not need a pillar as it is between two lifts (which themselves should count as "pillars" in this example). I would say length 3 and longer should follow the current pillar rules.
