RegExes wont help you as they can operate only on tags of single given track.
Oh well... guess I'll stick to not tagging the bonus tracks differently.
if it's that important to you, you could use a custom tag like "sort year". Then your template could use $IsNull(<Sort Year>,<Original Year>,<Sort Year>)
I'm not clear on what this accomplishes, or even how to do it?
Can't you just use Year for the year the bonus tracks were released and tag the same files with Original Year to match the original album release?
Then the naming template keeps the files together, but you can still see which tracks were released at a later time.
I hadn't mentioned this at first in order to keep things simple, but I actually use the following template at the moment:
<Album Artist> (<Original Year>) <Album> $If(<Original Year>=<Year>,,[<Year>])\<Artist> - <Title>
Where <Year> also appears if it's different from <Original Year>. So your suggestion wouldn't work for me.