Doubtful. I'd bet a lot of the people who can do this work wouldn't be interested in a whiteboard interview. If you're fishing in a small pond for people that can maintain your legacy code, there's going to be a lot less bullshit in the process.
Whiteboard interviews have their place, I'm sure. This isn't one of them.
I'm pretty sure with these kinds of systems, if you can repeat about 10 nouns relevant to working within a mainframe environment like this, you're hired.
I've considered it, seriously even. I could go off, contract for large hourlies, probably get paid a lot more than I do now as a more junior exec with a lot less stress (and when I first moved into management, I had the same thoughts). If you have the skills and reputation, this is an area where you write your own ticket. If I were closer to retirement age instead of in my mid-30's, I think I'd be doing it.
Whiteboard interviews have their place, I'm sure. This isn't one of them.