Is it possible to access plugin data from a custom feed template?

The solution, for anyone else interested, was to clear cookies and the cache:

clean_user_cache($current_user->ID);
wp_clear_auth_cookie();
wp_set_current_user($current_user->ID);
wp_set_auth_cookie($current_user->ID, true, false);
update_user_caches($current_user);
if( current_user_can( 'mepr-active', 'memberships:1,2' ) ) {         
  echo 'Hi ' . $current_user->user_login . '! You are a member';
} else {
  echo 'Hi ' . $current_user->user_login . '! You <strong>not</strong> a member';
}