But, visually, this would be more like MPEG-1 (per-frame) artifacting as opposed to MPEG-4 (vector-stream) datamoshing.
Artifacting can be pushed even further through oversaturation. The effect you are looking for as far as I have experienced it in Avidemux, is a result of making a cut that is not on an I-frame in Copy mode, so that some of the frames in the following group of pictures are not complete. Using any graphics software, you can export a JPG image-sequence at very low quality. As long as the codec is willing to decode the video stream with missing frames, data moshing will result. In the absence of frames, the codec attempts to fill in missing frames. The MPEG-4 codecs specify these different types of frames, which, like you say, is like a kind of motion vector. This article explains the relation between the different kinds of frames and how they're combined, but I think it's just missing frames in general and the codec's attempt to reconstruct: I have played MPEG-4 videos with large megabyte chunks missing, and gotten similar results - these would probably be missing any or all of the frame types.
