At the moment I think, that the best way will be to install XP in IDE mode and to switch from the IDE to an AHCI driver from within a running XP. My idea is to modify the AMD AHCI driver v1.2.1.402 to make it usable with Windows XP.
Unfortunately there is no AMD AHCI textmode driver (incl. TXTSETUP.OEM file) available for DEV_4391 AMD SATA AHCI Controllers.
Can you give me the link?