+1 to the feature.

A closely-related use case is customizing the message displayed by error(), which is normally the usage string followed by the error message.  I wanted to append instructions on how to invoke --help, and implemented it this way for CPython's regrtest:

http://hg.python.org/cpython/file/6ee721029fd5/Lib/test/regrtest.py#l205

Also take a look at how regrtest formats its usage string as another use case to satisfy:

http://hg.python.org/cpython/file/6ee721029fd5/Lib/test/regrtest.py#l9

It seems like many argparse customizations take the form of "override this method."  Would it make sense for the API to be for customizers to override string-returning methods like make_usage() and make_error() (and that accept a dictionary)?  That may give a bit more control than a format string.