Hah, It's possible in theory, but workable
only if directories where the original tracks came from never moved. A bit limiting !!!
What to do if they were moved ?
Another tag required to indicate the track was a compilation track [Compil track]
Another tag to indicate its original path [Compil track Path],
which could be filled by a copy from [Filename(path)]
A smartlist /expression that checked that current [Filename(path)] was the same as
[Compil track Path]
If so, then fine else list track, this will be the list of orphaned compiled tracks.
This could be simplified if MC (in the next iteration) could check whether there were any library references to a track before moving it. If so then update those references in the library with the new location. This might also fix moving cue files
And combined with meep's suggestion below ...
.. to be able to left-click an empty line in an album and have an "Add Alias..." option. That option would allow me select an existing track from the library which would then be virtually added to my album.