Heh. This is great! I'm stock full of good ideas, all I have to do now is see how evil I feel when they show up! One of the poor players has even commented (via e-mail), that these house of Mammon guys seem ok. My biggest issue is how not to to a TPK and end my campaign - or to relax and let it happen.
My gut reaction is to lock them in the room, cast darkness, drop lotus vapors until they are unconscious, then place a cyst in each of them. Afterwards, a round of torture, then perhaps animation or slavery.
Stick with this, these "cysts" get put in them, then they lose all recollection of what happened. Still in this House of Mammon, they get offered a job to do something for these people. You already have one PC saying they aren't so bad, so throw in a little gold for bribery and they should bite.
In reality they are carriers for something these people need to transport. So the PC's think they are off to do some great mission, in reality they are really just transportation vessels for what has been placed inside of them. Along the way you can drop hints as to what is really going on.
Are you talking necrotic cysts, ala Libris Mortis? That'd be rather cruel. Warranted, maybe, but cruel. Definitely some sort geas to find and return the second brain would be in order, though.