I like the idea of using something like Markdown to write GUI descriptions. But I don't think pressing a meta-language into Markdown is the way to go. Better use the same principles and create your own language. The principles are: text is formatted nearly the same way a plain text email would have been formatted, overview of the grammar is more important than complexity.

Don't be disappointed though if the result is not better than the solutions using XML, JSON or YAML that already exist.

Maybe something like funciton would add a new perspective to it: http://esolangs.org/wiki/Funciton