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.