Awesome tool you have going here.
I'd be interested in programming an add-in for Microsoft Word.
I read at http://wiki.languagetool.org/missing-features that doing this would require using CGAPI.
THB, I have no idea what that API / library is, but even without knowing what it is, I think I can safely say that you don't need it.
You can run LanguageTool as a server on your local machine and send queries to it via HTTP. This is exactly how the extension for Firefox works. You send an HTTP request and the server returns XML. I haven't tried out other plugins, such as the one for Open Office, but I suspect they work the same way.
If this is true, then I see no reason why it wouldn't be possible to program an add-in for literally any application that queries the local server the same way. I've written Office add-ins that query remote servers using HTTP. Why wouldn't it work with a local server?
What do you think?