In mods such as Extra Utilities or Minefactory Reloaded, there are items which when used on mobs or animals would "put it in" the item. the item could then be used again and the mob would appear. I don't know how possible this would be with plugins, but could an item that has this functionality be added to the server? It would make the transportation of animals or mobs much easier and it would be a way to remove money from the eco.