Right, I have been meaning to check this for a while but never got around to it.
I have a Syba SD-SATA-150R in my Digital Audio G4. It is a card that uses the Sil3512 chipset and like other Sil3512 cards with suitable eeprom, can be flashed with the wiebetech TCS1-1 firmware.
I know it can be flashed because there is a forum thread in another language based in another country that google translates very badly. It has a link to the firmware, an screenshot of System profiler with the card being used in OS X but as far as I recall I never read anything about booting off the card.(at least nothing translated into english)

I was looking for something to do just now and I remembered that I wanted to see if my card was in fact bootable so I poked around on google for a bit to try and find what wiebtech cards were bootable in Macs.

Turns out the Sil3512 based WeibeTech TeraCard is in fact bootable. After verifying that I was actually using the same firmware as the bootable card(my memory has been bad recently), I booted my G4 off a Leopard install disc then installed to the SATA drive as a test.
It boots! laugh

This is incredibly awesome.
This is the card I got:
Others should work as well but not all. I have a Sil3512 based cheapo noname card and it seems that it cannot be flashed at all, even with DOS flashing programs. That seems to be due to the components they used on that card though.

Anyway, this is how I went about doing it.
Insert the card into Mac.
Boot up.
Run the flasher program.
Follow the prompts and select the Firmware option.(it looks like Firmware option = Mac and BIOS option = PC, I might be wrong, I did not extensively test aside from that if you use the BIOS option it is no longer usable in OS X)
Check System Profiler for card recognition.
Shutdown and connect drive
To be fair, you might not need to do that extra reboot between flashing and connecting the drive. I just did not have the drive in my case at that point of testing.

TCS1-1 flasher is here:

The WiebeTech site says that this card should support hotswap but I do not recall having that work. So far I have had to connect my drives while my Mac was off for them to work. However, since it is bootable then I would want to boot off it simply because it means I can remove my older and more likely to fail PATA drive from the case and completely sidestep the LBA48 or whatever issue that forces me to partition my PATA HDD then use an enablery-mabob to access the rest of it.

Also of note. My Quicksilver 2002 would not initialize the video adapter or even chime with this card in it no matter what slot I put it in or what other cards are in it. According to the macgurus forum this is a known issue with some Silicon Image cards and some Digital Audios and Quicksilvers. It is not related to the WiebeTech firmware on the card or the fact that the card is flashed! Looks like at this point nobody knows the real cause, only that you need to try another card or another Mac. :P

With regard to the hotswap and non-boot issues.. YMMV.

Either way, this is frikkin great!
I still want to get more cards and do some testing.

WiebeTech also has firmware for their Sil3112 based card (TCS2-0) so it looks like I might have to try one of those next. wink
When I do I will let you guys know, unless someone else has a spare 3112 card on hand already.

It is about time we had this posted somewhere in english. ::)