No one has implemented something like that (on this board), because individually, it is very much easier to do without special classes etc. But on a community wide level, I think this sort of thing is pretty much mandatory, to help small projects and people getting started. I've been meaning to do something similar. At the moment the ethos of Box2D is orientated at bigger projects, where you will want to implement details like this yourself to tie in best with the rest of our system. But lots of people who build their projects around Box2D rather than visa versa, at least for the Box2DAS3 which attracts less, uh, involved developers.
A simple controller scheme should be pretty easy to put together. I was concerned that having GetShapeList, GetJointList, GetControllerList etc would get too confusing, so was pondering some unified system expanding on Joints. After all, your described "spring" controller would actually require two bodies, and have a lot of joint like information, while not actually requring the velocity or position solvers to step in.