The repcached patch adds replication support to memcached. I would suggest to add a USE-flag "replication" to net-misc/memcached and enable/disable the patch based on that decision.
It does not apply to 1.2.4-rc1. Could you please push upstream repcached to integrate with upstream memcached?
It applies flawlessly with 1.2.2, which is the current version in portage. Not good enough for a first shot? Nevertheless I will inform upstream to release a new patch.
I expect that upstream will release 1.2.4 sometime in the next few days to a week. The upstream mailing list is lagging a bit in the archiving spot.