Woops, realized that there's also a "Black-Briar Lodge" page, which seems to be far more incomplete than the one I just posted. Can someone replace one with the other? Decker12 (talk) 23:12, November 16, 2011 (UTC)
At first I thought that this lodge could indirectly be connected to the Black Briar questline which takes place during the main quest line of the Thieves guild. You help Maven of the Black Briars several times in that questline. I returned to the Black Briar Lodge at a later stage during the game after having finished the Thieves Guild questline completely but the Mercenaries do still attack on sight. This I guess either rules out that you can enter the Lodge as a friendly or is a questline glitch that has not been fixed yet by Bethesda.