Very nice. Have you considered RST  (or another markup language)
for the design document formatting? It's readable as plain text,
github displays some of these natively (I think at least RST and
Markdown), and RST can be "compiled" to PDF or HTML (e.g., Python uses
it for their documentation and generates output with Sphinx ).
Also, being plain text, it's fairly easy to merge proposed changes to
sections of your design document.
The main downsides are that formatting is limited (probably not an
issue--an asset, in fact--in something like a design document) and
that tables can be a royal pain (it's WYSIWYG, so you frequently need
to massage the table structure if you change data).