This is now good enough for real work:
https://github.com/lassik/scheme-pandoc

It's currently used to convert Scheme Surveys
(https://github.com/schemedoc/surveys) from Markdown to SXML, and could
be used for more things on Scheme.org.

Getting Pandoc to do fast batch conversions was tricky. After
benchmarking the Scheme pandoc library with help from the Chicken
maintainers and discussing the issue with Pandoc's author, this tool
came out: https://github.com/lassik/pandoc-tar

In case any schemers want to use the pandoc library (which calls
pandoc-tar) for some project, I'm happy to help you get started.