Hi,
This is a plea to allow passing arbitrary parameters to pandoc, and on a per file/frontmatter basis too. The original github issue that led to making pandoc an external helper (here) touches on the question of arbitrary arguments, and that point was rejected. Somewhere else someone also raised a security issue, due to the potential of running arbitrary code.
I want to ask again the feature of passing arbitrary arguments, because current pandoc is like the neglected third child in the helper family, you can’t do 
with it currently 
.
You can only use it for mathml, while it could generate literally any exotic output from any exotic markup hugo doesn’t process natively.
A friend of mine died a few months ago and left me his intellectual property to manage, including a few books. I intend to create critical editions, and output both TEI XML and HTML+RDFa (the latter for a static website) from a pandoc basis (much fenced divs and bracketed spans). Or more likely “Djot” , the new kid in the block). Getting all that from renderhooks would be a nightmare.
I use a shim but this sucks, because the semantic structure of the books requires different lua filters or default file. So the only practical solutions currently are either a very long, hard to read lua filter branching depending on the metadata, or just invoking pandoc before hugo. Both displease me immensely.