Isso foi discutido nos fóruns do EC2.
Destaques, direto da boca da Amazon:
if you do manage to hibernate the OS your Instance will still be in a running state i.e. you will be paying for the instance.
how do you plan to wake up this instance from it's sleeping state.
This really does not sound like it will work, We would recommend that you configure your instance to automatically launch all the services you need at boot time and use the stop/start feature of EBS backed instances as a solution to your problem.
(ou, em linguagem simples - não se preocupe. Não lhe poupará dinheiro, e provavelmente não funcionará de qualquer maneira)