The reason we do not offer a traditional IP/port is because anyone on the same server as you could read your memcached data. This would be a security risk which is why it's a socket that exists only inside your own accounts Cage meaning only you can access it.
I want to clarify one thing the recommended extension is memcached and not the memcache extension. The memcached extension is actively developed:
The memcache extension is not:
The last release is almost four years ago.
Looking at their code directly I believe we can make this work with sockets. What you can do is use the following:
The reason I'm coming to this conclusion is
They are calling the PHP extension and the documentation for addServer is here: http://php.net/manual/en/memcache.addserver.php and it accepts sockets on the host and you set the port to 0.
That should make everything work when you utilize the Memcache extension in PHP 5.6. Unfortunately looking through the phpbb forums there is no support for the memcached extension meaning you cannot utilize PHP 7.