For the campaign module it's hard to explain to people how they are supposed to use macro's to get content from their website into the campaign.
The Wysiwyg Fields module has almost the same functionality as the macro used in the mailchimp format. You can select content, choose a view mode or display formatter and add the markup to the wysiwyg editor. Would it be possible to make the campaign entity fieldable so we can leverage the power of Wysiwyg Fields?
On another note:
I don't really like the way mailchimp campaign tries to leverage the editable regions of Mailchimp itself. I understand you want Mailchimp to be good as a standalone module. I would have rather seen us using a content type as a newsletter with a self created view mode for the table markup.
How about also allowing the user to reference one single entity instead of using a Mailchimp template? You could allow users to choose to edit the standalone regions OR just select their fully created content in one click while still using the Mailchimp filter on the markup.
PS: the filter should also check for contextual link tags that might sneak in when embedding entity content.