So after the rsync update that got pulled in today I got to wondering why I needed rsync after all, given all my repos including gentoo are set to git sync and I'm not using rsync locally ATM, tho I have in the past.
Turns out the only thing pulling in rsync is portage (I've been running a null @system for years, if it'd otherwise be pulled in that way instead of just as a portage dep), and for portage it's a PDEPEND.
So for portage's benefit I setup a null-package rsync ebuild in my overlay and merged it, replacing the real rsync. While I've obviously not tested for long yet, I don't /believe/ rsync's used to pull sources or anything, so I should be good to go.
Of course that implies that instead of hard-coding portage's rsync PDEPEND, it should be made an on-by-default USE=rsync and IUSE=+rsync based PDEPEND, thus this enhancement request bug. =:^)
(If there's already an open request for this my bugzi google-foo was insufficient to find it...)
We also use rsync as part of the FEATURES=install-sources implementation, so we'll have to add a note about that in the USE=rsync documentation.