I should add a little bit of a correction here. Make sure you change the modelid to 14 before you apply the 8.1 update. Otherwise you'll get the notification that says "This update is not compatible with your computer." As long as you have the empty .img file to load the OS files, you can take that route and watch as it prepares the necessary files and folders. After this is done, make sure you go to Special>Shutdown. You can now delete the older disk.img. Before launching the Macintosh desktop, rename whatever you called that empty .img (now containing the Mac OS 8.1) to disk.img.

Every time you add a new game, you might want to open basiliskii.cfg and add to each line: cdrom /home/pi/RetroPie/roms/macintosh/nameofgame.iso (or nameofgame.img). This way, the .iso or .img will show up as a disc on the desktop. You just need to double click on each disc, find the executable, and double click it. That should go through the install process, but sometimes you might have to drag and drop a/several folder(s) into a single game folder where your other games might be stored.

Any idea if a Mac version maybe made?