Query the posts by custom field

Hmm, the best you can do is add your own shortcode, so you’ll probably have to deal with these limitations: http://support.mailpoet.com/knowledgebase/hook-to-add-your-own-shortcode/

Yes, unfortunately, we can’t offer any more help than that, because we’re slowly stopping the development of the current version of our plugin, in favor of MailPoet 3: http://www.mailpoet.com/mailpoet-version-3/

So whatever modification you’ll do right now, probably won’t work in the future.

Oh yeah!

MailPoet 3 will definitely be more extensive and allows much more customizations, rest assured!