With today's complex stack of multiple platforms in most enterprises, a good security researcher, IMHO, should be fluent with both worlds. Mainframes are where some of our most critical data is stored. When you pull up your account balance through your bank's website, there's a good chance that value was read off a mainframe.
Mainframers are old-school. They don't believe in public disclosure or open security models or public audits. If you go through the DEFCON and BlackHat archives, there's not much mainframe research out there. There's just a small community of mainframers on the Internet, but it's a significant part of the world's infrastructure. The mainframe world is a crazy alternate reality. (I know, because it's my day job.)
Phillip Young, the guy who owns this Tumblr project, has made some waves in this community. His talks are a great place to start. Here's a few resources to get you started:
[0]: http://mainframed767.tumblr.com/
[1]: http://bigendiansmalls.tumblr.com/
[2]: https://media.blackhat.com/us-13/US-13-Young-Mainframes-The-...
[3]: http://www.slideshare.net/bigendiansmalls/security-necromanc...
[4]: https://defcon.org/images/defcon-22/dc-22-presentations/Youn...
[5]: https://www.youtube.com/watch?v=Xfl4spvM5DI
[6]: https://www.youtube.com/watch?v=5Ra4Ehmifh4
Also, IBM.com has a wealth of documentation. (They have terrible SEO though.) Checkout the z/OS RedBooks and manauls there.