MarcC's post in 404 error on all pages, even Home was marked as the answer
Correct, mod_security in CPanel. I'm still looking into the issue but so far it looks like there was a false positive in some encoded third-party cookie data. Can't share the settings/config atm. Mod_security made the decision to 404 these requests.
MarcC's post in First Steps to Making New Admin Tabs? was marked as the answer
Hm, yeah, just making the tab is dead easy. I just made a new page under the hidden "Admin" page, published it, and I had a new tab. But it is asking for a process to associate with. So what I would recommend next is reading Soma's post, here: http://processwire.com/talk/topic/1272-new-page-nav-in-admin/#entry11276