You may have better chances with a Gigaset GX290.
A few guys already reported, to have SFOS running on a Gigaset GS 290. The GX290 is the Rugged-version of the GS290 with slightly different hardware-specs (i.e. Display, Memory).
But Gigaset released the same sources for both devices open-source-program.
So it seems most likely, that the port that runs on GS290 would work on GX290-devices, too.