"[A] key reason that the DOL has expressed concern with brokerage windows appears to be its perception that, by offering a brokerage window, a plan fiduciary can effectively abrogate its responsibility to contour and otherwise choose an investment menu, by making available to plan participants an extremely wide range of unspecified investments. Arguably, the DOL's concerns with the potential appropriateness of brokerage windows generally led to a disclosure-related approach that could have had the effect of making windows costly and difficult to administer and otherwise implement, or even completely unworkable in a number of cases."