When you compiled SDL, did you set your CFLAGS to "-DKOBO"? I followed the SDL.sh script in the xcsoar github repo.
Code:
patch -p0 < $PATCHESDIR/SDL-1.2.15-kobo-new.patch
CFLAGS="-DKOBO" ./configure --prefix=<whatever> --host=arm-linux --disable-video-x11 --disable-cdrom --disable-audio --disable-opengl --disable-json
make
make install
And I have dithering! I did a hard float compile, but I could easily do a soft float for you if you'd like.