I too have a 6950 and the same asus board and have managed to boot into mountain lion with no resolution problems or white screens, albeit I had to take out take out the 6950 as the resolution was terrible with it in. I still have to use "GraphicsEnabler=No" as a bootflag however. I think your problem lies in your DSDT file. The pre-made one you've downloaded for BIOS 1101 or something rather than version 3402 which you are currently running.
First thing you need to do is locate a patched BIOS which will eliminate the need for NullCPUPowerManagement and such things. So here is the patched BIOS file -http://bios4asus.blogspot.co.uk/. A small write up of info for the patched BIOS's is here -
http://www.tonymacx86.com/dsdt/43486-asus-1155-patched-bios-repository.html
After you've patched your BIOS you need to make a custom DSDT using this guide
tonymacx86 Forum • View topic - [GUIDE] Creating your own DSDT for most boards. Ensuring to use the "P8 (generic) BIOS 3xxx" patch rather than the specific one for your motherboard!! and for the audio use the "ALC892" patch which will be in the "Misc."section.
Now go through the UniBeast bootable USB creation AND tick the "Legacy USB support" which will hopefully sort your problems with USB. It won't break anything if you don't need it anyway. If your dual booting with Windows on the same hard drive then you'll need to apply this patch to the usb installation drive -
OS X Mountain Lion GM MBR Patch - Downloads - OSx86.net (The GM, or Golden Master build, is the same as the final release.
Now turn off your pc and remove the HD6950's and plug your display into the port on the motherboard's backplate. I will edit later and attach pictures of my BIOS config for you.
Now go ahead with the install, booting with "GraphicsEnabler=No" for the bootflag (I wouldn't bother with the "-v") Once the install is done it will restart your PC and you will need to still boot into the usb drive from the BIOS (pressing F8 at startup and then selecting your USB drive) and then select "Mountain Lion" once your in the bootoader, again using "GraphicsEnabler=No" as a bootflag. Then run multibeast (this is where I've not entirely figured out the correct process) but basically just select "UserDSDT Install" (putting the DSDT you made earlier onto your desktop at the same time) Then check on "system utilities" and then go through the relevant sections within "Drivers and Bootloaders" using this spec sheet for your motherboard will most definitely help -
ASUS - Motherboards- ASUS P8Z68-V PRO.
As I said before I still have to boot using "GraphicsEnabler=No"as a flag. I still don't have the HD6950 working for graphics or working sleep and am using USB audio so if you make any advances on these I would greatly appreciate any help.
John
