Message subscribe to new content

Hello,

I'm trying to understand how the message subscribe module could handle sending message to users when a new node is created. For the time being , flags are usable on existing entities, so I don't see any way I could use flags to mark content types as being "subscribable".
For short, I need to define a toggle for each user indicating whether the user subscribes to new content. (using a boolean field for example). Then, using rules, I imagine creating a reacting rule to create message and send it through message_subscribe_send_message(). But how to tell message subscribe module, that the selection of users should be based on user toggle field instead of flags ??

Thx fo any feedback !