First success: Using VLC, I transformed one of the old mpg sheep into an avi file without recompression (just chose AVI as the container, and told it to copy the original video stream). electricsheep --read-only 1 --standalone 1 --parasite 1 opened an mplayer window which looped the sheep.

Now I just need to automate that transformation to convert my whole 202 flock in-place and it should be fine. I'll cope with that later and post my solution here.

/edit: As the VLC converted files were not playable by Totem, I thought I'd better convert them using a different tool. ffmpeg -i xyz.mpg -vcodec copy xyz.avi produced files playable with both Totem and mplayer, which seemed to be an improvement over the VLC conversion, but then electricsheep could not read the files ("av_interleaved_write_frame: Invalid format").
So I reverted to VLC. for file in *.mpg; do vlc -I dummy $file :sout='#transcode{acodec=none}:duplicate{dst=std{access=file,mux=avi,dst="'${file%.mpg}.avi'"}}' vlc://quit; done converts all mpg files in the current directory to avi files (for mass conversion, find will be a better solution, but I'm still just experimenting). I did this to a folder of 23 files and started electricsheep again. It basically works, but for the loops there seems to be a fraction of a second missing (the animation jumps), and for edges there are ugly mpeg artifacts as if there were missing I-frames.

If there is anybody else interested in mixing old 202 and new 234/244 sheep in his/her client, please feel free to follow the steps I've taken and experiment further to get better results. For now I will give up and rather try to get an old client version to work, which will at least allow me to watch 202 sheep.