You are already answering it yourself a little. iFrames are really bad practice and can through-off search engine bots. If you say the content in the frames are all the same a few alarm-bells go off as well. Why would you want to use frames anyways? maybe you can clarify that a little or show us a the example so we can make the advice a little specific what to do next.

a 302 would suggest the page will be back anytime soon since its a "temporary redirect"
While a 301 suggest the page is moved. So personally i would use a 302 so that the page your landing on keeps the juice but the user will be redirected to a login screen for example.

I would suggest to noindex those pages if they are really hidden for search engines. (noindex,follow)

Interesting reads:

• Save your website with redirect on Moz.com
  and - SEO Advice discussing 302 redirect on mattcutts.com