I am having a similar problem. Your example above works if we have knowledge of tags of the outgoing message, but most of the time, we do not have such information beforehand. In such cases, all we have is if the message to be sent is text-only or has html part as well.
For example WP notification emails are all text-based by default. I tried setting up a filter on 'mandrill_payload'. In the filter, I checked if $message['html'] is set to ascertain whether the email was text-only or not. Unfortunately, it appears that text-based or not, $message['html'] always contains the body of the message and $message['text'] is always empty. Please fix the plugin so that $message['html'] is empty for text-only emails and populated otherwise. So that we can do necessary operations within the mandrill_payload filter hook.