I mostly use pandoc for processing HTML and Markdown files with some occasional epub generation. But sometimes I find myself looking at pandoc to convert between markdown, tid, and HTML for various reasons.
For example there is this md2tid.lua filter, but I find it doesn’t quite create a tid that imports nicely into TiddlyWiki. I think it’s important to distinguish between converting the underlying markdown syntax and merely using tid files as a container for markdown. The later seems more useful since then you are not needlessly converting the underlying syntax.