I believe you could use
output:
blastula_email:
includes:
after_body: my_footer.html
To include some html content in the <div class="footer"> part of the email.
I guess it behave quite the same as for a html_document on this header, footer thing.
An, example
---
title: "text_to_html"
author: "ThZ"
date: "30/10/2020"
output:
blastula::blastula_email:
includes:
after_body: footer.html
connect_footer: TRUE
---
# Something
Some content
```{cat, engine.opts=list(file = "footer.html")}
<p>This is a footer</p>
```
You can obviously generate your footer.html file outside the html file to provide in the YAML header.