It is important that we all have the same picture in our heads before we go on. Here is "my" version:
A mockup based on the discussion so far, the acs-subsite work Lars did and some old CMS pages -
http://blesius.org/projects/openacs/acs-subsite/
(there are a lot of goodies in the existing CMS package)
A major thing that is still missing from the mockup is the ETP interface. The path to that would be the "Edit" link on the index CMS page:
http://blesius.org/projects/openacs/acs-subsite/cms/