As far as Mechanus goes, I know that there at least used to be a Demigod called Primus, the One who was directly served by the Modrons.... I think they might have written him out of 3.x with his polygonal henchmen, though. =(
In 2nd edition certain gods like Mystra lived on Mechanus but they changed it.
"that nighted, penguin-fringed abyss" - At The Mountains of Madness, H.P. Lovecraft
When a man decides another's future behind his back, it is a conspiracy. When a god does it, it's destiny.