I've just set up an experimental job to try this to one of my repos:
https://jclouds.ci.cloudbees.com/job/EXPERIMENTAL-mirror-apache-to-demobox/
So from a technical perspective, at least, this seems to work:
https://github.com/demobox/jclouds-test-mirror
https://jclouds.ci.cloudbees.com/job/EXPERIMENTAL-mirror-apache-to-demobox/4/console
Quick, too: 1min 24s.
Whether we feel the security risk - the ability for someone to use the
deploy key to add bad stuff to the mirror (not the source!) repo - is
a sufficiently big problem would be a question for the list. It'd be
OK with me...
ap