This was surprisingly easy. All I had to do was make sure the asciidoctor gem was installed and use .adoc as the file extension. Hugo already supports asciidoc; one thing I did find confusing, was that if I wanted to run asciidoctor from the command line on one of the Hugo files it would fail. Turns out you have to pass -a skip-front-matter to asciidoctor to tell it to strip out the front matter section that Hugo uses.