It hasn't been done because its outside of the scope of design for rsync.
It's meant to sync arbitrary filesets in which many, if not all, changes are
made out of band. It's decidely non-trivial to implement in that mode
unless you're willing to accept a certain window in which your database may
be out of date.
But, in a situation like PAUSE, where the avenues in which files can be
introduced into the file sets is controlled, it does become trivial. It's
the gatekeeper, it knows who's been in or out.
so the requirements for the Solution To The Problem Which Solves A
More General Problem Than The Immediate Problem And Will Therefore
Make Whoever Sets It Up A Hero include a replacement for the current
mirroring technology stack that is tailored to mirroring distributions
possibly including on-demand caching and expiration and that is
trivial to install -- something like
perl -MCPAN -e 'install STTPWSAMGPTTIPAWTMWSIUAH::Mirrorsuite'
nohup nice nice perl -MSTTPWSAMGPTTIPAWTMWSIUAH::Mirrorsuite -e
'mirror cpan.org .'
