Interessanter Fakt am Rande: Der Mann hinter Old Bones Publishing, Matthew Eager, spielt ausgiebig Xoth mit MYTHRAS-Regeln.
Well, I do not know about "extensively", but I am one of the players in a "Spider God's Bride" play-by-post game, which is awesome. :-)
Playing in that campaign inspired me to write "Savage Swords Against the Necromancer". Currently in development, I have a second S&S adventure that takes place in another city-state in the same game world. Unfortunately, that scenario will not be ready soon, though. I have several Old Bones projects in development presently. Nowadays I have far more ideas than time...
On the DTRPG page for SSATN, I was asked whether a full S&S campaign setting/game world would be released, based on what is hinted at in SSATN. My reply was no, I had specifically developed that adventure _without_ a well-developed setting, to make it easier to drop it into anyone's Conan-esque world. That said, as I release more of these S&S adventures, some details of the world will be revealed here and there, and the setting will come into better focus.
My philosophy of writing background is, "Less is more." When I see a huge setting sourcebook, my eyes glaze over. I would rather give 5 pages of flavorful background with lots of questions left open for the GM to develop, than 500 pages of exposition that feel like a second-year history assignment...