I came up with this idea when I made some rough plans on how to build a smoothly running, basic fortress without too many potash makers running around and putting their noses in other dwarves business. I didn't want to tangle with the population cap since I consider that cheating, and simply sacrificing the peasants to Armok would prove to demanding since I would need to fill the ground with coffins. Instead, unwanted, sub-par immigrants get their profession tag changed to "scum" and are denied access to the fortress. Then I build a ghetto for them.
So, theoretically:
I seal off the land surrounding my fortress (it's more a tower than a fortress. 7 z-levels high, each floor a grand bedroom for my original dwarves). I give the scum some basic plump helmet farms, some workshops and a meeting area. Obviously the conditions in the ghetto are poor. This way I keep the scum alive long enough to make a decent amount of coffins. And since entrance to the ghetto is sealed off with a bridge there aren't many things there to hurt them. Besides the tantrum spirals of course,
and they should bring much enjoyment to my 3 hammersquads, who are prone to excessive violence.
Now, I wouldn't want too many of the scum dying at the same time. I would rather have a steady but gentle stream of tantrum spiral deaths. This means I have to give the scum some bare necessities in order not to rampage too hard. Would plump helmets and booze be enough for the scum to slowly succumb to suicide and murder, or should I take more things into consideration? If I send in my hammerdwarves when things get out of hand, would them killing the enraged dwarves be enough to re-establish temporary peace? Also, scum who prove themselves in the ongoing tantrum spiral olympics will of course gain access to the military and live a life of luxury.