I found a nice solution for playing flv files in slowmotion. You basically play the original video and store BitmapData Objects while it`s playing in an Array. Than you can just show these stored BitmapData Objects in at a given intervall slower than the original flv is playing. You just need to store the difference between what has been showed and were the original flv playhead is. This way the RAM increase is not too big.
In a final implementation one would layer the Bitmaps above the actual Video.
The whole thing is done in Flex but the same technique can be applied to Flash 8 projects as well.