This is the script for compilations, which sets Original Album and Original Artist:
$if($or($eq(%compilation%,1),$inmulti(%releasetype%,compilation)),
$set(originalalbum,%album%)
$if(%albumartist%,$set(originalartist,%albumartist%),$set(albumartist,[compilation])$set(originalartist,[compilation])),)
Then you also need to put originalalbum and originalartist in Preserved Tags, so once it's set it doesn't get changed later.