Well, I was thinking more that I would want to set up the phone so that the baseband OS is only operational at the time I decide I want to make a phone call. Would something like that be possible and practical?
There's a GSM hardware switch - https://hackaday.io/project/19035/log/60071 - so this is the way to go then, simple as that =) So far, the software is going to assume that the modem is on at all times - however, I'll solve that in the next PCB revision.