If one train is over booked as compared to another train in same route which may have seats available there must be some reason for the same. It could probably be that the other train with vacant berths is much slower or timings are inconvenient.
Unless and until it is an emergency or passengers want to compulsorily travel, i dont think passengers will prefer other options.
Also i dont think this will work in high traffic routes since all the trains might be full.