OK, I have fixed the cropmarks. The code from the 550D pretty much worked, with some minor tweaking. The new code allows you to add up to 9 cropmarks to your folder, and ML will find them and cycle through in the menu. Known issues with the code:
CMs seem to not draw where the audio monitor resides
When you switch between CM files there seem to be some remnants of the old CM image
I don't know how to update the code repository, so I have attached the update zebra.c file (can someone help here?).
Ben