Ok, so I am back on Tarma and am about to make what I (probably vainly) hope is my final attack on the MPlayer versus Windows Media Center Edition DVR problem.
The DLL in question that I am led to believe will decode this file is stored in C:\Program Files\CyberLink\Shared Files\VideoFilter\CLVSD.ax on my machine. We shall update the codecs.conf file and see what happens, no? 
And failure. I edited the file, copied the DLL into C:\mplayer\codecs, and tried to play a video. What happened was this:
========================================================================== Opening video decoder: [dshow] DirectShow video codecs Warning: DS_Filter() no such class object. (DLL=CLVSD.ax, r=0x80040111) Failed to create DirectShow filter ERROR: Could not open required DirectShow codec CLVSD.ax. You need to upgrade/install the binary codecs package. Go to http://mplayerhq.hu/homepage/dload.html VDecoder init failedCannot find codec matching selected -vo and video format 0x20525644. Read DOCS/HTML/en/codecs.html! ==========================================================================
And thus, the investigation continues.