Gappie,
Since my previous posting I reloaded Media Center thinking I must have done something wrong during the re-install process. Long story short, after this latest re-install, the first thing I did was to restore my library from the backup. Now all of the user defined fields are there! That's the good news, however, data only shows up for one of the fields (one I labelled "Opus"); there is no data in the other user defined fields.
Given this result I'm "assuming" that for the "Opus" field I must have configured it as "Store in file tags (when possible)", so that the data was actually stored in the file, and was "recovered" during the backup process. I must have defined the other fields differently, and therefore the data was NOT stored in the file, and is therefore lost.
Is this a reasonable conclusion? So in future all user fields should be defined such that the data is stored in the file?
R