TheOptimizationIdiot - 5:46 am on Apr 22, 2013 (gmt 0)
Well, the only way to do it so it's accessible is like was suggested with noindex,nofollow (I'd personally serve it in the server header subdomain-wide) and I'd add rel="canonical" pointing to the corresponding page on the main domain.
Also, similar / not exact clone doesn't really matter. You can reason with us all you want and say it's for some internal purpose and not exactly the same, but the algo's totally unreasonable and a near duplicate you launch and try to get indexed could range from totally ineffective and "ignored" by search engines (at best) or it could really hose your main domain if the algo decides it's to add extra pages and more chances at visitors (spam).
There's no "duplicate content" penalty specifically, but there are plenty of "looks like spam" based penalties and possible negative ranking impact scenarios from near duplicates and duplicates (which I believe increase when it's not www v non-www exact duplication) I wouldn't use a second near duplicate subdomain for anything personally.