I'm a little flummoxed. Patching relative_uri in the
sphinx/util/__init__.py to re.split on ("/" + os.sep) sorts out the
static generation. But it seems to introduce a couple of problems with
the web-server version of the docs. One is that links end up without a
category: http://localhost:3000/windows/ instead of
http://localhost:3000/using/windows/. The other is that, even if you put
the URLs in by hand, they don't work whereas /using%5cwindows *does*
work. Which suggests that the windows-y path is being stored somewhere
in the pickle files.

So now we're stretched two ways: the HTML needs /-delimited names to
work; the pickled files need \-delimited. I'm still looking at it, but
if anyone has more knowledge that I have (couldn't exactly have less)
then please feel free to chip in.