bizhawk.multiclient --dump-type=ffmpeg --dump-name=foobar.avi --dump-length=1000
type is one of 'vfwavi' (doesn't work), 'ffmpeg', 'jmd', 'nut', 'wave'
name is filename to dump to; might be auto-modified to add segment split names (_00, _01, etc)
length is length of dump in frames after which it auto-stops. if omitted, set to be equal to the length of the movie loaded with --movie, or if not that, it runs forever (can be stopped from UI)
zeromus