Pretty much all add-in IDE and SATA cards are seen as SCSI devices by the BIOS.
So no need to really disable anything. Just set your BIOS to boot from SCSI / add-in card and you should be set.
Yes but if I understand correctly, the SCSI card (whether it is really SCSI or another interface that merely appears as SCSI to the BIOS) would also need additional drivers supplied at install time, for the XP and earlier NT to install and run. Just like AHCI needs such drivers.
Although it's good that you mentioned this, because now that I think about it, the add-on card doesn't necessarily have to be PATA anyway. One could probably use an actual SCSI PCI card which has drivers already included with Windows. (Adaptec 2940?)
I wonder why you ask this question because you can set SATA devices to be seen as IDE in many BIOS options, it may be listed as 'compatibility mode. Also, it's possible to install AHCI (SATA) drivers to add support for newer systems on XP/W2K/NT4, you can also use nLIte to slipstream the drivers into a W2K or XP install CD.
What I am hearing is that new motherboards don't have the IDE compatibility option anymore, and drivers may not even be available for the latest chipsets. I don't have any motherboards like this, since mine are all old, but I'm wondering about socket AM4 boards.