This patch has a 'custom_help' which, with a default template, is compatible with 'format_help' (i.e. it passes test_argparse).  It also handles the sample template in this issue.

Due to long line wrapping issues, the 'Usage: ' string the test template has to be entered separately as a usage 'prefix'.  Indenting of long wrapped values (like usage) is correct only if the '%(...)s' string is at the start of a line.

I see this as a test-of-concept patch.