My last drive-by-software-hacking turned out to be useful at the end. I hacked up a basic QT RLE (aka Animation) encoder for ffmpeg, sent the patch to ffmpeg-devel but without further intentions to work on merging. Thanks go to Alexis Ballier for stepping up and refining my patch until it was polished enough.
With recent version of ffmpeg, you can create a 1280x1024 screencast for your running X session at display :0.0 with
ffmpeg -f x11grab -s 1280x1024 -i :0.0 -f oss -i /dev/dsp -vcodec qtrle /tmp/lala.movafter running configure
./configure --enable-gpl --enable-x11grab --enable-libfaad --enable-libfaacUnfortunately rgb555 encoding was removed, so you are only able to create videos rgb32 video streams (aka 24-bit color depth).