In case it helps anyone, the way I achieved this was to put single disk games and m3u files for multi-disk games in the root of roms/x68000, and place the images for the multi-disk games in a separate sub-folder (roms/x68000/multi) then write the m3u files to point at them.
The system ignores the sub folder and just the stuff in the root is displayed.
Root of x68000:
6b7f8744-7d12-48fc-abe3-c0ce2921bc79-image.png
Contents of 'multi' folder:
b0e9f1ad-1d6b-4241-86b0-2e4d45bc0db3-image.png
