In the latest development build of Reason you can restrict access to minisite pages. How does it work? After running the appropriate upgrade script, just attach a group to the page (you'll see a new "Restrict Access" option when editing the page.)
All pages in the hierarchy beneath the restricted page inherit the access restrictions, so you can use this feature to restrict whole sections of a site if desired.
If a parent page and a child page both have access restrictions, visitors will need to belong to both groups to view the child page.
Note that, at the moment, Reason does a separate directory service lookup for each group it needs to check the visitor against, so it might not be a great idea to create a deep tree of pages and attach a group to every page in the hierarchy if your LDAP or other directory service is poky.