I've just uploaded LibDsk 1.5.11 (http://www.seasip.info/Unix/LibDsk/), which dips a first tentative toe into the world of GCR-encoded Macintosh disk images.
This means that in the unlikely event that someone uses it to convert a Mac 400k / 800k GCR floppy image to a CPCEMU EDSK, you'll find the recording mode (byte 13h of the Track-Info block) contains a value between 0x10 and 0x2F inclusive, most probably one of these:
0x12: Macintosh 400k / 800k
0x14: Apple IIgs Prodos 800k
0x22: Apple Lisa 400k
Such disk images would also use the 'trailing bytes' extension to store the 12 'tag' bytes that the Macintosh associates with each sector.