I am happy to announce the launch of LocalCharts Docs, which is an instance of Hedgedoc, an open source collaborative markdown editor. There is no technical integration between LocalCharts Docs and LocalCharts; it simply provides a nice place to copy/paste markdown to/from.
I hope that LocalCharts Docs will be a convenient way of writing drafts for posts, and getting feedback and collaborating on them before posting them to LocalCharts!
Note that while in fact hedgedoc and discourse use the same library, markdown-it, to render markdown, they may have different settings/plugins. So there may be some slight differences in the rendering; make sure to double check after you copy/paste that it still looks good.