The router is loaded before the get_instance() function is defined, so you wouldn't be able to load the memcached driver for the cache library, even if you could load the library itself. You could build your own class to abstract the cache methods needed for your routing, though.
As for autoloading the Cache library with the correct drivers, create a file: /application/config/cache.php with the following:
PHP Code:
$config['adapter'] = 'memcached';
$config['backup'] = 'dummy'; // or whatever backup driver you want
$config['key_prefix'] = ''; // or whatever key_prefix you want
Then make sure the settings in /application/config/memcached.php are correct.
