I am guessing jonap is aiming to have some database-like table (excel sheet, text file, I don't know) that contains the artists and their birth dates, and that a virtual tag would query that 'birthdays' file.
Unless the Additional Tagging & Reporting Tools plugin has some brilliant functionality that I am unaware of, I don't think that is currently possible.
edit
It brings to mind similar questions/requests regarding 'genre' if I recall correctly.
Some users would like to store the genre for an artist somewhere only once, and then have that matched whenever a song by that artist is selected or playing.
So I can see some value in the general idea.