So, it seems that I was laboring under some faulty assumptions. I didn't realize that the surgery which had originally been discussed was not the surgery my brother was having. Yesterday's procedure was minimally invasive - they went in via a small incision in front of his ear and went in through a small, natural opening of the skull. Details are sketchy, but about 75cc of spinal fluid spontaneously ejected from the cyst just from having some pressure relieved. I don't know what the source of the pressure relief was or if this is even an accurate description. But apparently the surgeon said that after measuring the fluid, they were happy with that reduction and didn't do anything to further drain or disturb it. So that's why the procedure was so short. A CT scan later in the day showed a sizable void in the area that was previously all fluid. All reports are good.
The downside is that even on morphine and Dilaudid, he now has the mother of all headaches, and this will probably continue for the next week or two. I can only imagine that as the brain tissue decompresses after being deformed by such a large mass, there would be a lot of pain. I guess this is a classic case of "you have to get worse to get better."