Adrian,
OK, I'll test my mac ingest again, but I clearly remember having to rename the avchd related files back to their original names so that I could get them into iMovie as a camera archive import.
I guess this potentially begs a slightly different angle, that of ingesting avchd files from camera cards - could they be renamed by photomechanic upon ingest as other movie formats can? I'm not aware of DSLRs using avchd format, but panasonic 4/3rds cameras certainly do.
Photo Mechanic has no knowledge of the AVCHD 'format'.
Another complication that could arise in your settings may be that flattening the directory structure could make it impossible for some software to recognize the 'format' of the files.
At this time, Photo Mechanic is suited for dealing with single movie files or photos, and not a set of related folders with various files in them. I suggest either preserving the directory structure, or using the Finder/Windows Explorer to copy the AVCHD folder and its sub-folders to your system.
-Kirk