Mac OS 9.1 is officially the final OS supported on the Power Mac 7200. I've got a new PCI graphics card and PCI USB card on the way and so I'll need Mac OS 9.2.2 for full compatibility. Below details how to achieve this.
Mac OS 9 Lives
I had previously downloaded 9.2.2 from here. It's known as Mac OS 9 Lives and it comes as an ISO with Apple Software Restore and a disk image of a partition with 9.2.2 fully installed. To install the software you only need to run the restore and it'll turn a partition on your local machine into a bootable system image.
..of course, I installed this and rebooted and got the standard This startup disk will not work on this Macintosh. I assume from here you could hack the system folder and edit the gestalt ID matching code (as was done with the 68k) but I instead chose to use OS9 Helper.
To get my machine booting again, I had to find a boot disk that worked fine on my hardware to switch back to the 9.1 startup disk. My old Presto PPC boot disk wouldn't work on this PowerPC... so I had to guess how to boot from a CD. Turns out, with the 9.2.2 boot/firmware, that holding down C at boot will startup from the CD.
Official 9.2.2 ISOs
This wasn't going to work either, so I started researching and heard out about...
There's quite a few links that explain how the Power Mac 7200 owner should use OS9 Helper to hack OS 9.2.2 into submission. The basic idea is to download OS9 Helper and run it on your machine.
This application will not hack/fix an installed version of OS 9.2.2. Instead, it requires that a version of 9.1 or a patched version of 9.2.1 reside on a local hard disk. It will then update this to the version you require.
This stopped me from using the partition I'd just created with Mac OS 9 Lives and therefore made me use my previous 9.1 startup disk.
The app requires that the relevant Apple OS Update for the version you wish to install be downloaded locally. I didn't try the download links available... instead I searched the net. Finding this was much easier than expected; it turns out that my own ISP still has a cache of Macintosh software! iinet's public FTP located here has a whole swathe of archaic Apple software to download!
Once you've got the required updater, re-run OS9 Helper. I quickly found out that you can't just jump to 9.2.2, you'll need to follow the upgrade steps and install 9.2.1 first. Installing 9.2.1 via OS9 Helper is a breeze. Once done, restart your Mac. It should boot back up to your desktop without issue.
Once booted, set up 9.2.1. You're now ready to apply the 9.2.2 update. Do this via OS9 Helper once again.
And now you're done!