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One way of marking a string for special wrap handing is to make it an instance of a subclass of 'str'.
This patch adds class _WhitespaceStyle(str), and 5 subclasses corresponding to the possible values of the CSS whitespace:.
 Normal
 Pre
 NoWrap
 PreLine
 PreWrap 
Together they define methods:
 _str_format() - apply % style formatting
 format() - apply the py3 {} style formatting
 _split_lines() - style specific split_lines (may or may not pass through text_wrap)
 _fill_text() - style specific fill_text (again without without test_wrap)
All return a text object of the same class (as self). This make it possible to apply the % formatting to a string, and then apply the wrapping, without loosing class information:
 Pre('sample text %(default)s')._str_format(dict(default='Boo'))._fill_text(30, ' ')
This subclass information is lost when the string pass through other 'str' operations, for example '\n'.join(). I needed to add _str_format because % formatting is applied to them before text_wrap.
The HelpFormatter has:
 _str_format() - all the previous % formatting instances
 _split_lines()
 _fill_text()
These delegate the action to the respective white_space classes, or use the (default) Normal subclass if the text is a plain 'str'.
test_argparse.py has 2 test cases that use the 'Pre' class to replicate the behaviour of the Raw...HelpFormatter class tests. Undoubtedly it needs further tests to handle all of these new classes.
I haven't made any doc changes yet.
I wrote these classes based on the descriptions of what the CSS options do, but I have not tried to compare the handling of sample text. I can also imagine users wanting to refine the wrap handling further (e.g. http://bugs.python.org/issue12806).
I intend to write test files to show how these new classes could be used in the various issues that I listed in the previous post.
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Since I had to collect the % formatting cases into one _str_format() method (to preserve class information), I am also exploring the use of Py3 {} formatting. 
Py3FormatHelpFormatter - a new Formatter class that redefines _str_format() to handle {} style formatting (if present). I put this in a separate class because there is a slight possibility that existing code has text that might be confused for Py3 style formatting, e.g.
 help='help text {default: %(default)s}' 
I think the issue of using Py3 formatting was raised and rejected. So I'm not committed to including this feature.