We want to offer this “e-book for susbcribe” deal in the middle of the blog post, as additional related info,
so regular readers, who are subscribed already, will be disappointed they cannot get this piece of info right now, while they read this post.
Finally I implemented this feature as a little standalone plugin, on client side only,
by replacing your form submit ajax handler by my own, so i just show previously hidden bonus area with download links, when subscription form was successfully submitted (we use single opt-in strategy), and then i call original handler to do rest of ES job.
May be later i will publish this plugin.