There is a pre-defined custom tag "Original Year" available in the custom selections.
Maybe 'Year' for the compilation release and 'Original Year' for first release would work.
It certainly would and makes more sense than using [year(yyyy)].
The main reason I want to do this is because I rename folders as [year] _ [album] and - because it picks up the year from the track - this means I end up getting multiple folders of the same album if I do automated folder renaming with musicBee (sure I can hardcode the album year into the path but would be nice not have to have to think about that in the future...)
I'm still learning what musicBee can do and this has led me onto discovering the excellent Tools/Tagging Tags/Library Report.
With a bit of playing around you can create a spreadsheet that you can filter to only show albums that include songs with different years. Very useful.
I swear if I look hard enough I'll find a 'free beer for life' button in MusicBee somewhere - and it would actually work...