As far as I know, you don't have to sign the MPN right away. In fact, I do believe you can just accepting your award at the level of the school, and then wait as long as you want. (In UG, I accepted 5k in Federal loans for my senior year but did not ask for them to be disbursed until 3 weeks before graduation...) So as far as I can tell, you can accept with a school and keep it in your pocket. Or at least my school permitted this.
My suggestion: ask. Call the direct loan program or the people at FAFSA directly. They should know (or at least tell you whom it is better to ask).