True, I read chrisnighhtley's point, but forgot to address it . smajdalf alluded to using the MOSI pin possibly in a similar manner. I am hoping he comments on why he suggested that pin. Regarding your mention of the radio - the radio draws 900na in "power-down" mode, - if it can be kept in that mode as long as possible the radio is a low load on the sleeping system.
Thanks for the comment!
Dan