Stop redirection for admin functions

I’ve seen similar issues to this, but not quite the same.

I’m setting up a WordPress installation for someone. We don’t want the admin functions (“wp-admin/*”) available via the public front-end, which would have the canonical URL. Instead, the plan was to use a back-end IP not accessible from the public Internet, with a separate hostname (and therefore URL). I don’t want them to have to override the canonical hostname in their DNS setup.

I’m not at this point concerned about the SEO ramifications (there will be only one public hostname), but if there is a way I can do this so that rewriting the URLs to be canonical is ignored on a particular instance, that would be ideal.

I tried the simple “Disable Canonical URL Redirection”, but redirects and links still seem to use whatever the canonical URL happens to be. In addition, the login session seems to get lost (cookie issue?)

Any ideas?