Hi, Tumbleweed -
Though my hair isn't long, I don't think it would have made a difference. My CK mask was made with me lying on a specialized table with a narrow head support that has anchor stubs where the mask is secured. Before forming, the mask material is dampened so it's soft & flexible (it's sort of like an open-weave cellulose cloth) and then it's draped over the face from hairline to chin, and extending down the sides of the face to the anchor points on the table.
The technician (Larry, if you go to Stanford) molds the fabric to your facial features as the mask hardens. It's quite open, structurally, and easy to breathe through. You can see through it too. (and it makes a nifty Halloween costume)
It leaves your crown open and doesn't wrap around the back of your head at all, so I see no reason why long hair would be a problem as long as the back of your skull fits securely into the molded "pillow" they custom make to support and stabilize your head.
You can always post an inquiry on the Cyberknife site - I'm sure it won't be the first time they've been asked that.
Best wishes with your adventure!
Adrienne