Mine is the stock HP z400 board, model #461438-001. It has an Intel chipset (82801JR), so I figure that it should work. I've flash the latest BIOS updates and tried your suggestion of reverting back to the default BIOS settings and then making sure AHCI is enabled. Still no joy. I've tried using Unibeast with Lion and iBoot with Snow Leopard Retail DVD. Other suggestions found that I've tried include verbose mode, safe mode, PCIrootUID=0, GraphicsEnabled=No and all the combination of them. I've also tried the HD in all the SATA ports and using PS2 Keyboard and Mouse in order to avoid using USB (for iBoot obviously). With Unibeast/Lion, I was able to boot into the installer, but it couldn't see the HD, so that's why I tried iBoot/SL thinking if I could get that up and running, I could just upgrade with Unibeast.
I found someone who compiled a custom DSDT for this motherboard, but I'm not sure how to load it in the install process, only after using Multibeast.
Any suggestions would be appreciated! Thanks!