API / Notifications for Custom Capabilities

I’ve read here: http://www.s2member.com/kb/building-an-api-notification-handler/

Payment Notifications are also triggered whenever a Buy Now purchase for Independent Custom Capabilities takes place. Check your Dashboard for more information about Independent Custom Capabilities.

However, what I am wondering is how can I customize the email notification sent to the user when a custom capability is purchased?

For example, I would like the notification to include all posts that require the custom capability to be accessed, i.e.:

Thank you for your purchase!
Here are the posts you can view with your updated account:
[link]Post 1[/link]
etc.

I realize the above would require coding, which I am comfortable with, but is this even possible without hacking s2 core? If it is, where would I start?

Thanks!