Is it possible to run OSX/MOS/Linux directly from the SSD connected to Sil3132 ?
Yes - if you have working Sil3132 with MOS (with better luck than ernsteiswuerfel).
But you have to place booting image to separate partition, visible by OF of computer - i.e. on SATAI internal storage in case of G5 PCIe - MOS installer do it for you.
I have similar configuration on my Peg2 - boot.img is on IDE CFcard, and whole system on PCI SiI3114 controller + SSD.
Linux for sure works - again, grub or yaboot must reside on OF-readable partition.
OSX probably no - you must have driver for it, and I don't know if it exists for ppc OSX. And even if you find the driver - again you have to separate kernel + drivers to OF-readable partition and rest of the system to SATA2 drive.
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad