These are some links that may help.
I also need to rewrite the acs-subsite group and user management pages for a personal/volunteer site I'm working on so I'd be very interested in seeing what you and others have done.
I think the acs-subsite admin pages out of the box have very confusing UI. For instance, the simple and common task of adding a new user to a subsite takes 8 clicks, often with obscure choices.
It would be interesting to collect all the customizations people have done, and try to distill the commonalities.