Indeed, SiI3132 is a PCIe-to-SATA2 controller. It might work with a PCI-to-PCIe bridge in an OS running on Pegasos II, but you won't be able to store boot images on a drive attached to the card, because SmartFirmware can't see behind bridge chips. This issue would also be there if you found a PCI card with SiI3132 as the bridge chip would also be there, just on the card itself in this case.
It's a pitty. This is the same situation like with Promise SATA 300 TX4. It is native PCI, without bridge, but driver is third-party only.
Pegasos II 1.33GHz, AmigaOne X1000, Powermac G5 Quad, iMac G5, Powerbook G4 and Z10PE-D16 WS
MorphOS, AmigaOS, Debian, Ubuntu, OSX, NetBSD, Win7