I think Locale will be your best bet, but I don't know of a plug-in or even a program that will check to see if you have coverage and adjust the radio accordingly. Great idea, maybe that could be implemented as a Locale plug-in in the future or something.
Actually, I don't think there is a way for an app or anything else that would do what he is asking, mostly because of the design of cellular access from what I can tell. If I recall correctly, all cellular phone access technology will 'try' to get a signal, and once it does, it will try and sustain enough power to ensure service until it can't do it at all. I am not sure if it will simply 'passively' detect it if you had an app that put the phone in Airplane mode because it is no longer trying to make a connection, there for, cannot sense if you are in a live area. Your closest, next bet would be GPS reception detection where it goes, "oh, you are within this area." which would also require you to be on the outside for it to work within reason.
From my understanding of Locale, and how I have it setup for me, it would try and triangulate where you are at, check the conditions and go from there. In his case, it would be best to have timed conditions as well as location so it will default to that state for the most part and then once he leaves that area under the time conditions, it will flip back to normal state.
