A modified New World ROM at a minimum.
The interrupt signals in the ANS are wired differently than how they are in the PowerMac 9500. I don't know, but I'm pretty sure the ROM contains information that maps the various interrupts directly to specific PCI slots and devices. On the ANS, that mapping is scrambled. So if one installs a PM9500 ROM in teh ANS at some point the ANS is going to grab the wrong interrupt handler.
Apparently at boot time according to reports of folks who have tried it.
The 2nd paragraph is informed speculation. The third paragraph is what I remember folks reporting.
in theory you should be able to boot OS X directly on an ANS without any need to swap ROMs since the ANS does run OpenFirmware
as OS X bypasses any need for a Toolbox ROM image and boots directly from OpenFirmware including on OWR macs
the only things you would need to do is figure out what NVRAMRC patches an ANS would need, being technically and Old world ROM machine im sure will have plenty of OpenFirmware quirks that need patching before OS X will boot, like all other OWR macs need
and then add the ANS model identifier to the plist in the PowerSurge Platform expert kext for OS X (since I dont think its there by default, but I know the ANS is very similar architecturally to a 9500 so id like to think the same platform driver would work)
then in theory it should boot... (although if your trying to run Later then 10.2.8 you will need a suitably patched kernel like my 604 10.4.11 kernel)
I have sadly not see anyone try, but it would be fun to see someone try