well... this may not be EXACTLY what you were looking for, but another forum member and I created a script that plays back movies at a reduced resolution depending on how high the bitrate is. You can get that here and run it from the command line with: "mplayer-opt <video file>"

However the bitrate of a video corresponds to how many bytes of information is processed per second and the only way to play a video at a lower bitrate is to slow the whole video down and play it in slow motion :P

So your best bet is a lower RESOLUTION decoding solution