And the answer was a simple one: I forgot to enclose my options in an array in the $backend* arrays, like this:
$backend_system = array(
'name' => 'Memcached',
'options' => array(
'servers' => array( array(
'host' => '127.0.0.1',
'port' => '11212'
) ),
'compression' => true
)
);