Seems like it really isn't what was intended for that field:
TYER: The 'Year' frame is a numeric string with a year of the recording. This frame is always four characters long (until the year 10000).
You can actually enter more characters in that field (I've used other programs, like MP3Tag, Roxio's "Edit Audio Tags" (which for some reason gets invoked on my machine when I right-click on an MP3 in Explorer and select "edit song information"), and I've stored "YYYY-MM-DD" in there. Both MP3Tag and the Roxio program accept the hyphen and save and display the now-extended date, but the extended date does not show in Explorer.
Winamp will not allow the entry of anything other than a number in that field (using it's "file info" editor).
As far as using some other variable character field for holding this extended information and mass-copying the field transfer automatically, I can't help you there. But I don't know of any other field than the "big ones" which will be supported by portable music players or other Windows media player programs, which you could transfer this data to and still have it visible somehow.