[shaggylive]

well, I think there's a call blocking app that will block outgoing calls too, so maybe a mortscript to check available minutes and when out/low, then change the apropriate setting / file in call block app..
sorry i'm not more help, but this would be a good project. I read about the call block apps but i've never actually tried one. I'm sure there's one that has database file that can be easily manipulated.