Would this be very complex to program? Maybe this is a quick and dirt feature only for mods?
Well, you clearly see that the object of this feature would be to distribute the slots to more people. It's not perfect, but theoretically DC++ only has to stockpile MB transfered for every user who is currently using a slot. When he makes 100MB, DC++ stops the user for 10 seconds, enough time for other users to claim that slot. If nobody claims it, the first user regains the slot and download continues.
I hope this idea didn't come up before in the countless threads on leeching