On my second playthrough I got the chest, but ended up killing the two Zhent guys I rescued from the bugbears (after they told me about the hideout and the pass phrase.) I kept the chest locked and returned it to the Zhent leader (don't remember her name.) The leader was happy and paid us. She also specifically said I was 'a friend of the Zhent.' However, the trader (don't remember his name either) still said that I couldn't see the 'good stuff,' it was only for friends of the Zhent. This is bugged.
I'll have to try the method described above by DarkRob316.
