The easiest way to visualize this is an elevator that moves up and down on a track that's built into the side of a cylinder. At this point I have three bones.
The arc bone describes the track that the lift follows along the wall. It's head is in the center of the cylinder, and it's length is the radius of the cylinder.
The train bone is parented to the arc bone, but points in the opposite direction. It will be a little train that drives up and down the track and holds the lift. It will always be pointed toward the center of the cylinder.
The platform bone is attached to the end of the train bone. It needs to remain at the same angle like a platform would.
What I want is some way to create a control bone that I can move up and down along the z axis such that the platform is always at the level of the control bone. Is it possible to set up something like this? Would I have to rig the platform in a different way in order to make this possible?