It is now mining! Just 4 of the 6 GPUs are installed but it is cranking away on Zencash and at the expected speed. The last hurdle I had to cross was Windows not recognizing the 2nd through 4th GPUs. Reinstalling the drivers solved the problem, but it seemed like a rather inelegant solution. I hope I don't have to reinstall the drivers yet again when I plug in the 5th and 6th cards...
All in all it was a frustrating process getting this board working, but that was almost entirely due to the lack of documentation and outright wrong instructions on the Alibaba listing page. Now that I've done it once it will be much easier to set up my other board... Once I get GPUs for it, that is...