[justin66]

Treasurydirect is great and everything you say is true, but the poor user experience and the commonly reported difficulty involved in getting help if there is a problem make it more appropriate for personal finance than business, I would think.

More generally the approach of using a cash sweep account in conjunction with t-bills held with a custodian bank seems like pretty sane advice. I don't think I'd emphasize "one bank," but otherwise, sure.