I confess that I have recently watched Hugo and the movie provided fresh inspiration for the development of some sort of swarm bot based on mechanical rather than electrical means. This swarm robot website was pointed out to my by my Network Science 675 professor and is really interesting as well. One approach they use is to enable the robots to read light as a kind of pheromone for emergent property formation. I like it.
Could you use hollow styrofoam balls that somehow interact with each other in a directional way? Would magnetism be a useful information or energy transfer medium? It can be complicated to work with but perhaps the movement could be constrained in two dimensions somehow or along a track with the emergent behaviour falling along a single distance-between vector.
If anyone has seen something like this, I'd love to learn more about steam punk robotics.