Update
Use the piped version suggested by user Christian Long:
pandoc -t latex mydoc.md | pandoc -f latex --data-dir=docs/rendering/ -o mydoc.docx
I know this is late in coming, but I’ll be assuming people are still searching for solutions to this three years after the original question — I know I was.
My solution was to use LaTeX as an intermediary between markdown and docx (actually, I was converting from org-mode, but same difference). So in your case, I believe a one-liner solution would be:
pandoc -f markdown -t latex -o mydoc.tex mydoc.md && \
pandoc -f latex -t docx --data-dir=docs/rendering/ -o mydoc.docx mydoc.tex
Which might get you closer to your goal. Of course, Pandoc has about hundred arguments it can handle, and there are probably ways to make this prettier. It has also gotten quite a few updates since you first posted your question.