Managing multiple sites with my own custom theme

Hey, so I am thinking about consolidating a few wordpress sites I am currently running to use on common theme, with a set of customisable options. That way I just have one theme to keep up to date, simple.

I've looked at using a service cuch as Infinite WP or WPRemote to help bulk manage all these sites. these services claim to allow you to upgrade plugins and themes in bulk, but I'm not really sure how this could work with themes. To my mind this would only work if I published my theme to the main Wordpress theme directory, that way these third party services would pick up the latest version number, and a zip file with all the code. I don't really want to list my theme in the directory though, as I want to keep it for personal use - is there a way I can tell Wordpress where the 'master' theme files and version number can be found?

Thanks for any help or advice!