And if I study our iMac G5 iSight Schematics, there is another interesting findings:

iMac use SATA connected HDD and ATA connected DVD.
But in real, used Southbridge have two SATA ports, and only one is connected.
If anybody wants (and he finds appropriate pins on motherboard) I have the scheme how to connect it. It needs four additional condensers, but it is not too complex.

Then you can connect or modern SATA optical drive, or second SATA HDD (with both disks 2.5" size to fit to one 3.5" bay).
Of course, it is only HW point of view. I am not sure if OpenFirmware or OS recognized device which was not taken into account.

Southbridge is Broadcom, type (yet) unknown, but can be ( at least partially ) identified by PCItool, device ID. Apple codename is SHASTA. But for SATA port addition no identification is needed.