BRILLIANT!!!!

Dos1 is mistaken here - since all modems and also our Cinterion modem are heavily tivoized nowadays (means you CANNOT load firmware that's not signed by modem manufacturer's crypto key), we actually could ask Cinterion to simply erase "the firmware" on the modules we receive from them, and we provide the officially supported firmware flasher and firmware (C)&signed-off Cinterion.
However let me say that I have a hard time believing that our customers would appreciate such a move. And it sounds like sophism towards FSF rules "the firmware must not be changeable" - "so fine, we don't have any firmware in that device, so flashing a new one is not `changing the firmware´ since there is no firmware to change"
We could send a device without modem mounted to FSF for evaluation - if it actually was worth the effort (which I still doubt)
It all boils down to FSF not approving devices with GSM radio, no matter what you do, since FSF is not about privacy and user freedom but about Free Software. The approach to demand the impossible to force industry into the right direction is maybe idealistically a politically correct thing to do (though I think it's hybris), but it for sure fails for small projects like Neo900. Thus I'm not willing to worry further about FSF approval, it can't be done.

PS: on a sidenote - I wonder how many of the peripherals in Mr Stallman's recommended laptop still have some firmware that only nobody except the manufacturer knows about it and how to update it. Definitely all HDD and optical drives, all touchpads and mice and probably a lot of other subsystems have such "hidden firmware". What if eventually somebody - maybe even in FOSS community - finds out about a way to flash new random stuff to those controllers' firmware storage? Will the laptop then lose the FSF approval?

PS2: for now our modem has no way "anybody" could change the firmware, since we don't know about the flasher tool needed to do such firmware update, and for sure we won't ship the device with such tool.