Ok, so I've updated/patched the bios, which now allows me to install and boot without problems but the video is still garbled. So I rebooted in safe mode (thanks @MiniITX), and I'm able to get to the desktop in full 1080p, but with no graphics acceleration - meaning I can't do any videos and such but at least it allows me to play around with the driver options. When I check the info in "About this Mac", the Graphics correctly lists Intel HD4000, but with 0MB for graphics memory. I've tried the HD4000 enable options, i.e. Chimera, and then EFI string injection, but that doesn't seem to work either. One interesting thing I noticed was that in the System Information > Software > Extensions, I could not find anything related to AppleIntelGraphics. Another thing to note is that my motherboard BIOS does not have any GPU Memory related options, so I'm not able to configure that bit as it is required in the "Enabling HD4000" thread.
I still don't have sound either (my onboard doesn't work, so I'm using a discrete ASUS Xonar DG card), but I can get to this later I suppose.
Btw, @MiniITX, I'll send you my DSDT.