I am not sure but I had similar problem with my other site and I endup with below hack.
Same as /admin/ I wanted to redirect all pages under /some-category/ so I have created the folder on the same level from the root. And place .htaccess with 301 code and things are done nicely :)
So you can try by creating admin directory to your q2a and place .htaccess into that directory with whatever rule you want in it.
But I just wonder what and how you want to control admin section? by IP or what?
Pleset let us know if this hack works..
Note: Q2A never keep URL admin/ but it always add the page right after admin.. /admin/general etc..