The --help and readme both mention a -t / --to flag to specify the output format. The readme also includes an example of using pandcritic as a preprocessor. This is exactly what I want to to, but the flag does not actually function as advertised.

$ pancritic - -f markdown -t markdown -m m --critic-template /dev/null <<< "test"
No output file extension nor to-format specified. Default to HTML.


<div id="wrapper">

<p>test</p>

</div>

Note the -f / --from flag does work correctly for the input side of things, and specifying an output file name with -o something.md also works to auto-detect the to format. What cannot be done as advertised is pipe the output into pandoc or anything else as markdown.