Read Instructions Carefully
I edited the default Niresh Mavericks AMD7000Controller.kext, AMDRadeonX4000.kext. Install them with Kext Utility. You will need to install AppleHDA for ALC to get HDMI Audio working (seems like all ALC codecs work for this, Multibeast 6.4.1 has this, just choose one).
Edit your DSDT and insert the text from the injection file. You'll have to use IORegistryExplorer to locate where GFX0@0 or display@0 is. Look at the device just above it in the hierarchy (mine was PCE2@2). Find that (PCE2 for example) in the DSDT and insert the text before the last bracket for that device. You will also need to add the DTGP method at the top (Google this if you don't know). You may have to change the device you found just above your graphics device to PEGP (Find/Replace All in DSDT). I did this originally and have not undone it to test if you need it.
The injection file uses framebuffer Aji. I edited that framebuffer specifically for this card. The same edit is applied to Hamachi and Futomaki. Any of those should work the same (I was chasing my tail at one point and ended up editing all three of those).
I have this working right now with GV-R785OC-1GD. Please disregard the resolution shown in System Information. I have a DLP TV, it's hard to see from way back here. 1080p works fine for me, just too small from the couch.
If you are having Audio distortion, open terminal and type "yes > /dev/null" without quotes and hit enter. This is a cheap workaround for AMD systems, I've heard disabling AMD TURBO in BIOS may solve this as well.