Thanks a lot carpman!
The following codes seem to work fine:
$replace($directory_path($substr(%path%,12,999)),'\','|')|%filename_ext%$if(%subsong%,$ifgreater(%subsong%,0,|%tracknumber% - %title%))
$replace($directory_path($substr(%path%,12,999)),'\','|')|%filename_ext%$if($greater(%subsong%,0),|%tracknumber% - %title%))
The whole time I thought I needed something like "if length/amount of %subsong% is greater than 1, then list, else don't list", but when I just simply added '%subsong%' to the standard code for testing, I witnessed all singletrack files returning a %subsong% of 0! So that's when I knew the only integer for this code to work is 0!
So now multitrack files (with embedded cuesheet) are expandable just like directories, while singletrack files remain unexpandable.
Note: I haven't tested it with multitrack files with separate cuesheet.