Update theme without using repository?

Hello,

Is it possible to use the versioning/update ability for themes in any way other than making them available in the .org repository?

For example, if i’m running multiple WP installations and want to share a common theme across them all (and thereby customizing with a local, child theme)… and if that common theme were to change (via minor bug fix or large scale redesign) the best way to roll out that change would be to use the WP update function. But to do that, it seems the theme needs to be a part of the .org repository.

So is there another way? And if not, is there a way to make the theme we upload private? Seems private is not the WordPress way and I agree with that, but in this particular case, the issue isn’t a fear of sharing code… it’s a fear of unauthorized reproduction because the client is a content publisher (ie, newspaper).

Thanks in advance.