Is it safe to leave things in the abandoned shack chest and dresser after you join the dark brotherhood?
I've tried storing items in the shack too; it has not disappeared until now, though I highly doubt it will remain there forever like it does for houses.
I read that the game engine is resetting stuff once in seven days (in Skyrim time), so if you're not revisiting the shack in seven days, the stuff would disappear.
It is safer to keep the items in the guild instead. Or more preferably, in your own house.