I mounted a third fan in my case to blow air on the back of the GPU to keep memory temps down. However, it only spins up when GPU core temp gets high - which is relatively much easier to tame. Is there a way to either link the aux fan to the memory temp or set it to run all the time? It sounds like there is supposed to be some way of controlling and/or monitor the fan in X1, but I don't see anything.

As cpt00 has said, only way I see this being automatically variable is if your motherboard can see the Tjunction temp sensor.  Which depending on which model card you've got could be feasible.  I noticed recently that gpu-z and corsair dashboard can read my GPU temp/Tjunction Temp/hot spot temps with my FTW3, something that wasn't showing up when I had the xc3.