I'm not sure I get this question - I'm not talking about a white-boarding exercise. I'm just talking to the candidate about a technical project they have worked on in the past, the pain points, how they resolved them, etc. If that results in the candidate being flustered it's not a good sign on numerous axes.
> How do you hire people who are more technical in a subject area than your current staff, if your test bottoms out at the depth of the current interviewer in the room?
That is a _very_ different hiring situation. If you have no one in the company that is equipped to properly technically interview the candidate they obviously you can not properly technically interview the candidate. You'd likely have to leverage the connections of the management team to find the right person to lead up unknown territory.
I generally think software developers (I count myself among them) know far, far less about psychological assessment than they think they do.
Are you hiring sales reps or engineers? Why are you asking for an engineer to sell you on their past project(s)?
Interviews by their nature are about judgment, and knowing you're being judged by a complete stranger who holds the power over this potential path of your future is enough to get anybody flustered, especially a brainiac that just wants to work in relative peace behind their desk.
I don't ask them to sell me, I ask them to dive into the technical issues they faced and how they solved them. It tells me a lot more about how they will handle similar issues in the future than scribbling DFS on a white-board would.
I don't understand what you're calibrating for...