Can Cygwin Setup maintain a “mirror”?
NO. Cygwin Setup cannot do this for you. Use a tool designed for this purpose. See http://rsync.samba.org/, http://wget.sunsite.dk/ for utilities that can do this for you. For more information on setting up a custom Cygwin package server, see the Cygwin Setup homepage at http://sources.redhat.com/cygwin-apps/setup.html.