Figured it out (to some degree). WMV playback in Media Center uses whatever the "Default Format" is for "Shared Mode" on the device, while WMP puts the device in exclusive mode, and hence can play the WMV track directly. By making the Default Format "Dolby Digital", I at least got 5.1 from Media Center. Now I have three possibilities:
1. Find a hack to make Media Center always use exclusive mode
2. Find a hack to get "WMV" into the "Default Format" list for shared mode
3. See if AC3 spidifier will save me.
I didn't buy the SC-07 so I could do software conversion of audio formats! :)