FoxyCart v050+ has subscription-specific XML datafeeds. These allow vastly simplified subscription-based permissions to be implemented, but as yet no code exists. If you need this or are interested in building it yourself please let FoxyCart know in their forum and they will be happy to point you in the right direction.
Paid webuser subscriptions for MODx. Allows you to use FoxyCart subscriptions to manage webuser permissions. Useful for creating a paid members only section, paid blog subscriptions, paid premium content subscription, etc.
Could be modified to use a non-subscription product as well (one-time payment for unlimited access).
Paid Webuser Subscriptions / Permissions in the MODx repository
Support thread in the MODx forums