Is your membership site's functionality entirely web-based? Or are there downloadable components (like WoW)?
If it's all web-based, then is there any reason to make the addons downloadable? Or would they be web-based too?
If the addons are web-based, then they really just become features of the membership site, which would normally only be active for currently subscribed members.
If the addons have to be downloadable, then they could be made to interact with some aspect of the membership site, which would allow them to be disabled for non-subscribers.
you could create your addon that does a verification against a database hosted on your domain. If the code passed in has a status set to disabled then you quit the add-on