Monitor website (http) not accesible via webmin (server status tool)

I am trying to workout how to test that a site is inaccesible with webmin's tools... System and server status.

This is... an URL that should not be accesible to anyone except certain IPs or an http logged user which we do via nginx.

The questions is similar to this How to check HTTP response code of a secured website with Nagios? but for webmin.

The tool allows a head, get or post responses but when using head response regexp is disallowed so I can't seem to work out how to test. I've included this as a suggestion on the webmin project but perhaps there might be another monitor type to achieve what I am trying to do.