Actually, I just got it working with voodoohda 2.56. I had tried 2.60 or 2.61 (forget which, whichever one is in MultiBeast) and it didn't work at all. It made one of the SMC kexts KP on boot. Anyway I'm pretty happy with 2.56. Sound in and all that works great. Hopefully I can figure out how to make a disabler for AppleHDA so I don't have to carefully remove it in the future. Or, hopefully a future AppleHDA will support ALC892.

Now for power management... Maybe I'll sneak into a place that sells Mac Pros with a thumb drive and copy the power management kexts when no one is looking.