Re: How much protection does an add-on GSM modem give me vs. built into phone ?

John Case wrote:

the real trick I am interested is isolating (or at least controlling) the interaction between the baseband processor and the application processor. Using a computer with a USB dongle gives me that control ... would I have that same level of control if we had free software running on the baseband processor, or is there still additional bleeding possible simpy by virtue of being built into the computer ?

In a smartphone it's almost not possible to distinguish the "computer" from the "GSM modem" anymore, because of how the hardware is constructed, so yes.

what is the industry term for "making changes to application processor side of customers handset?"

Maybe you're looking for FOTA - Firmware Over The Air?

//Peter