I was thinking of longer-form posts to WP when I asked the question.
Mac seems to better served than Windows for this sort of thing (as it is for most things creative!). There's a few Windows ones but they don't seem to be as well supported, e.g. BlogDesk which hasn't been updated since 2009.
Have just found a WP plugin called DocxPresso, so I might try that with LibreOffice Writer. I'm familiar with Writer so that's worth a try.