As you can see, his binoculars were indeed aimed almost exactly at the zenith. He reports that he couldn't quite get close enough to the eyepieces to see the entire field of view, and the position was fairly uncomfortable. But reducing the angle 10 degrees or so made everything much easier.
I decided to repeat the experiment -- you can see the photo of that, too. I certainly got within 20 degrees of the zenith, and possibly 15 degrees. I continue to feel some neck and shoulder pain as I type this, but I'm pretty sure it will go away by tomorrow. The closest I can get to zenith with any semblance of comfort is about 45 degrees.
My preferred method of viewing the zenith with tripod-mounted binoculars is to tilt the tripod back, turning it into a bipod, as shown in the third frame. In case you're wondering, this is still a lot stabler than a monopod.
The moral is that people vary greatly in how much they can bend their necks. And the degree to which you can bend your neck will probably determine how happy you are with tripod-mounted binoculars. My own experience is that most of the things I want to look at are uncomfortable to view, and some of the things I want to look at are impossible to view.