If it is enough for you to distinguish original year and the release year of this release you could do something like:
%_release_year% will only be set if originaldate is different from date (the release date of the actual release you are tagging).
If you really want to use the date only if there is an actual remastered AR on MB you can’t use scripts directly for this as this information is not available directly. A plugin could be made to fetch this data and make it available as a tag.
Is the remastered AR used a lot? My problem with this AR has always been that it is a release to release relationship and I don’t know what to set as the original one. A remaster usually is based on the original studio recordings (unless those are unavailable). Now let’s assume the album was originally recorded in 2010 and has been released on CD, Vinyl and digital. All are based on the same final mix of the studio recordings, ideally each mastered slightly different to take care of the medium specifics.
Now there is a 2020 remastered re-release, which is also based on the same studio recordings. When I add such a release, which release do I point to in the remaster AR? The original CD, Vinyl or digital release? All of them?