Currently, if you want to use the Mencache handler, you have to configure ezpublish.yml so that it conforms to the example provided in:

https://doc.ez.no/display/EZP/Persistence+cache+configuration#Persistencecacheconfiguration-Memcache

On top of that, according to:

https://doc.ez.no/display/EZP/Session

...you'll also need to configure the session save handler settings in php.ini as documented in http://php.net/manual/en/memcached.sessions.php, to something like this:

session.save_handler = memcached
session.save_path = "localhost:11211"

Having the session configuration separate like that is not desired for maintainability and ease-of-use, so it should be possible to configure all required parameters on ezpublish.yml.