Experimented on 2nd hack. Works a treat on a GA-Z77X-UP5 (bios rev 4), with GTX580.
For those with 'Mack_kernel not found' error on USB bootup, remember to make things easier by ensuring your boot order AND boot drive in BIOS is set to the USB (or equivalent name) drive in FIRST order, then save and reboot, hit f12, boot & install from the Unibeast USB drive/stick, and u should be good to go.
If you're not using the Intel GfX onboard (and its disabled in BIOS) you may find it easier to boot with;-
-x -v GraphicsEnabler=Yes to get to the Yosemite install screen.
Once successfully in to Yosemite, try a basic Multibeast install first to test before adding/changing anything else, e.g.
Quick Start: No DSDT
Drivers: Disk>3rd Party SATA; Trim Enabler (check 10.10.0, if required); Network>Intel> AppleIntelE1000a v3.10 (checked - it works!)
Bootloaders: Chimera v4 (checked)
Customise: 1080p Display Mode; Generate CPU States; GraphicsEnabler=Yes, Hibernate Mode - Desktop; IGPEnabler=No; Use KernelCache.
Then click the 'BUILD' tab.
After the BUILD has finished you can Fix Permissions and reboot. Everything should be cool to go.
Note: All USB ports are working fine without selecting any further kexts after the basic 'No DSDT' install above.
Later: Change things like System Definitions/other kexts on the next Multibeast config run to test out other settings.