In the Page (or Post) admin in 'Custom Fields' add a new custom field named: pugpig_post_access
It can have the value 'protected' or 'free' (or be left blank):
Save or Update the post after this
In the edition's atom feed, the page should now display these changes:
This can also be done automatically using the pugpig_feed_post_custom_categories hook. See Pugpig Hooks for more information.