Power Macintosh setup in MAME git master as of this posting. NOTE: the driver's not yet promoted to working for several good reasons, so while this procedure works, running other software and System versions on the driver may not have great results. Also, this assumes you have the mac_cd software list items.

To start, create a blank hard disk file with your favorite method. On Linux, FreeBSD, or macOS, this command will make a blank ~500 megabyte image named "powermac.hdv":
dd if=/dev/zero of=powermac.hdv bs=1024 count=524288

Start MAME like this:
./mame pmac6100 -hard1 powermac.hdv pwrmaccd

After boot up it should look like this:
[Linked Image from rbelmont.mameworld.info]

Double click the Utilities folder and then double click HD SC Setup.
[Linked Image from rbelmont.mameworld.info]

Click Initialize and then "Init" when the warning comes up. You'll get the spinning beachball cursor for several minutes so now's a good time to turn throttle off in MAME (switch to MAME keyboard mode and F10) and go watch a YouTube video or something.

When it beeps that it's done, name the hard disk. I used "Power Mac HD", but the name doesn't matter.
[Linked Image from rbelmont.mameworld.info]

After that, click the Partition button and then click the Custom button in the window that appears.
[Linked Image from rbelmont.mameworld.info]

Click the small partition you just created (here called "Power Mac HD") and then click the Remove button and accept the warning.

Next click immediately below the "Mac Driver" partition and drag down to the bottom so you get a partition that takes up the entire disk.
[Linked Image from rbelmont.mameworld.info]

In the window that comes up, select "Macintosh Volume", and if the size displayed in the edit box isn't the full amount shown for "Maximum", feel free to change it to use the whole disk. Click Ok when you're done, then Done, and then Quit.
[Linked Image from rbelmont.mameworld.info]

Close the Utilities window and double-click "Install System Software" and click OK in the introduction window that appears.

Click "Install" to get the Easy Install option, which will do what you need. The installation will begin.
[Linked Image from rbelmont.mameworld.info]

When the installation is done, click "Quit".
[Linked Image from rbelmont.mameworld.info]

Drag the "Utilities" folder to the icon for the hard disk you partitioned and it will copy those utilities to the drive image. Do the same with the "Apple Extras" folder.
[Linked Image from rbelmont.mameworld.info]

Click the "Special" menu and select "Restart", which will automatically eject the install CD and start up from your newly installed hard disk image. That's it.
[Linked Image from rbelmont.mameworld.info]