I just wanted to mention that I'm happy to accept pull requests for pandoc-placetable. Especially, automatic calculation of column widths (if done well) and a table width attribue would be welcome.
Btw, pandoc-placetable supports a filter-less mode now (not pushed to hackage yet):
pandoc-placetable --file=foo.csv --widths="0.2 0.8" | pandoc -f json -o output.html