Thanks a lot for refreshing my memory about 0xFFFF. I used yesterday for something totally different but it saved one of my N900 with dead gsm (except the dead gsm is still dead, well...).
Flashing with the default flasher was impossible. It stayed stuck when trying to flash something to the modem, I believe.
With 0xFFFF, I could flash perfectly.

Back to the backup question, IIRC, I read somewhere on TMO that there is a question of image size or something. When you change it, flashing is not working any more...
But it looked very complicated to get to the point of something which did not work, so I abandoned the idea of something which would have worked.