Full steps to reproduce the modifications are included in the download. The gist of it is similar to what Bolle did for the 9250 in MacOS 9: binary edits to put the appropriate PCI ID in the right places.

ATI Resource Manager was the real trick for pre-OS9 compatibility: later versions crash at startup, but fortunately the last MacOS 7.6-compatible version seems to have shipped with undocumented support for the Radeon 8500 :-D

New listing is a 1.42/1 GB/128 GB SSD. It has a little bit of yellowing on the body but is otherwise in good condition. Mac OS 9 pre-installed and the auction has no reserve:
https://www.ebay.com/itm/126479563359

It comes with USB speakers, a period Apple mouse, and the power adapter.

3440x1440 via DVI on a G4 Mini 

(at 30Hz refresh rate--you can't win 'em all)

Is 9500/9600 not an AV Mac even though it's very similar to the other Macs listed?

No. 9500/9600 lack the AV board with audio and video inputs and outputs.

They also have 6 PCI slots and, IIRC, two PCI bridges, each for 3 slots. Adaptec 2940 is a busmaster card, so that also could be a reason why there was different firmware for different machines. Since 2940UW worked in G4 machines too, I think AV firmware was a special case. I don't remember much, but I think that video in AV Macs went thru PCI bridge and SCSI cards had to "play nicely" with everything else on the PCI bus.
I also vaguely remember that at some point Digidesign required Adaptec cards to be throttled down for compatibility reasons with ProTools (or was it Avid?) systems.

What's the difference between the AV and the non-AV version?

How about double clicking "PowerDomain 2940UW v4.1 Readme"?