Then there is no need for this firmware to actually connect to any known GSM network in the world.
But If you think that joerg_rw/OpenPhoenux/Goldelico are going to be responsible for users upgrading their firmware, then why did you mention that "it's better for user freedom to give him/her the ability to upgrade the firmware"? In this case, it's better to just source a modem with non-upgradable firmware or just blow some fuses so that it is not upgradable. Apart from increased legal protection, you would also get FSF endorsement!