I started a new plugin in the SVN Whiteboard:

org.apache.forrest.plugin.output.Markdown
"
Produce output in
<a href="http://daringfireball.net/projects/markdown/";>Markdown</a>
"mdtext" format. This enables use of Forrest as a tool
for gathering input from various sources, and generate
a consistent set of output documents.
"

I won't be able to help much, but at least it is a start.

-David