Tom,
there is a section in the CVS texinfo documentation about "Tracking third-party sources". The idea is to have a vendor branch in your local repository where the OpenACS releases are stored. Your own changes are put into the main trunk of the revision tree. If a new release comes out, you merge the changes since the last release into the main trunk.
A Web interface to this section can e.g. be found at
http://sunland.gsfc.nasa.gov/info/cvs/Tracking_sources.html
