Does your site use multiple store views?
If not, you can just create a second store view and have that use the development theme.
To prevent users from seeing it, just remove the store selector drop down (it’s in layout/page.xml).
You’ll might need to set your site to use store codes in the URLs, which could effect your current URLs if you are doing advertising or SEO. It’s in the admin -> system -> configuration -> web -> url options -> Add Store Code to Urls = yes
Hope that helps.