There are multiple interdependent cache types in accesslib:
1/ context cache - static cache, eliminates database queries when creating context instances - this could be a good target for MUC caching
2/ $ACCESSLIB_PRIVATE an array of several static caches used in access lib - it should be relatively easy to use MUC instead
3/ $USER->access - stores current user access data which includes users roles, capabilities in contexts where user has role, switched roles, temporary access, etc. - I do not think this should be touched at the moment because it is relatively fragile and more complex than other parts. Maybe role definitions could be moved away to some other cache type to make the session size smaller, but I am not sure if that would have any performance benefits at all.
4/ cache_flags - it is used as complex session data synchronisation mechanism, it is verified on each page access to make sure all changes from other requests are immediately propagated to $USER->access - it is not a simple key based storage and I am afraid the reliability of memcached storage is not good enough here
5/ List of capability names to verify that a capability exists ... currently a simple static cache.
6/ Getting role names and lists of roles (new code in 2.4).