"[R]egulatory guidance indicates that the choice of the broker, and negotiation of the broker's fees and other charges, may very well be a fiduciary act.... [DOL] has raised the question of whether there should be some responsibility for plan fiduciaries to at least monitor the usage of investments through the brokerage window.... [T]he participant-directed plan disclosure rules ... require that disclosures regarding the brokerage window arrangement, and the related fees and charges, be provided to all plan participants eligible to use the window, not just those actually using it."