Yes, I see what you mean, nice discovery.
First off, ftype does not say that it will create a filetype, just change or modify, so if you don't have MPlayer.Video in the registry, I doubt it will work
If you had the MPlayer.Video key, I'm no command line expert, but see if this might work:
assoc .avi=MPlayer.Video,avi 1>nul
assoc .mkv=MPlayer.Video.mkv 1>nul
assoc .flv=MPlayer.Video.flv 1>nul
assoc .wmv=MPlayer.Video.wmv 1>nul
Also the .mkv file extension has a Content Type........video/x-matroska and a Perceived type...........video which your script does not include. Same with the .wmv file type.
Isn't there an easier way such as going to Default Programs, selecting Windows media Player and uncheck those file types.
Now right-click on one of those extensions and you should have an Open With entry on the right-click context menu where you can select Mplayer from the list and check the box to set it as Default.