So, quick update. First up, looks like no Mac Pro ever made is going to meet the specs out of the box.
They specifically requires an AMD R9 290, which is Hawaii. ie, the generation AFTER 7970/R9 280X/D700.
And the SDK versions specifcally say after AMD 7xxx and "no multi GPU". So, no Mac Pro ever made meets this.
I just tried with a 5,1 sporting a single X5690, and it told me my CPU wasn't good enough. I followed the link and it told me that it was all about single core speed and apparently the age of an i7 3.46 makes it slower than the required i5 4590 3.3.
That being said, my guess is that they have put those standards pretty high to make sure the original experience is awesome, and not average. But I doubt they will have "locks" to keep mid level stuff out, and if they do, someone like Netkas will find a "fix". Stuff without enough VRAM may be excluded however by functional limits. As a "for instance" I can tell you from experience that Uningine Valley will crash and quit on Extreme HD if you don't have at least 1GB of VRAM. I could see this requiring more than 2 or 3GB, which would exclude all but D700.
I think I'm still the only one to get eGPU working on Windows with nMP but eGPU with other Macs will likely work as well. Will be silly if I have to put the 3.7 4-Core back in to get it to say "A-OK"
All in all, with the vast and sweeping compromises made for "thin and shiny" over function, Occulus Rift will not be putting a good light on Macs.