Can we extend e-newsletter template to include custom PHP or shortcodes?

We want to be able to dynamically include excerpts from a CPT into our newsletter and are wondering if you have any suggestions about how we could extend the plugin functionality for that purpose.

We are thinking we could extend the class Email_Newsletter_functions in our theme's functions.php file and add our own cases to retrieve curly-braced placeholders from our custom template.html file in order to populate them with our post excerpts.

We're not sure if that would work, and would appreciate your thoughts on this, or suggestions for other methods.