Currently it is required to manually perform an UPDATE query to update session data. This is a pain. For developers and MOD authors it would be nice to have an API for updating the session. I would suggest making it so:
$session->update_session($session_data, $session_id = null)
The optional session_id allows updating an other session, this however defaults to the current user's session.
Example usage (note that user extends session):
$user->update_session(array('session_page' => '/custom_page'));