Why not just change the text at /acs-admin from "ACS Administration is used to administer the site-wide services of the ArsDigita Community System." to "OpenACS Administration is used to administer the site-wide services".
Then on /admin just place a text on top of the page "[subsite name] Administration is used to administer the subsite services".
Small changes but that should help new users... that there is a differenct between the 2 of of them. Or why don't we just rename "OpenACS Administration" to "Site-Wide Administration".