This is the Markdown plugin for bbPress that I needed, but could not find. It replaces the standard editor with a responsive Markdown form that includes a preview and formatting help. The Markdown is parsed using PHP Markdown(Extra) and saved as a regular HTML bbPress post. The Markdown is saved separately for later editing. The plugin can be activated and deactivated without affecting your bbPress data, it uninstalls cleanly (deleting any saved Markdown code) and is multisite compatible.
Tip: You can give the plugin a slight performance boost if your forum is Markdown only or you don’t care that some non-Markdown posts are displayed slightly different than before (e.g. not turning newlines into paragraphs or no longer making URLs automatically clickable), by adding the following to your theme’s functions.php: add_filter( 'bbpmd_remove_output_filters_for_all_posts', '__return_true' );