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Are you aware of help2man? It can transmogrify the output of "your-program-that-uses-argparse --help" to a man page. Another solution is to use docutils’ rst2man to convert some documentation file to the man format.
On the other hand, this blog post contains code implementing a custom optparse formatter for man pages, which could probably be adapted for argparse easily, so it does not look hard to implement. (Note that the code has no licensing information.)
Steven, what do you think? Too specialized for the stdlib or small, useful new feature?