CHDs for software lists are stored identically to ROMs, folder named after the list with zips of the chds inside (yes, really).
Whaaaat... I thought zipping a CHD was a really bad idea.
Moreover, isn't putting in a zip file the same thing as putting in a folder, so that putting in a zip file in a folder is like putting in ./folder/folder and so doesn't work?
BY the way, mac_hdd works perfectly here with the last SVN build.
I have my hdd in software/mac_hdd/nameofthehdd/whatever.chd