thanks for your ideas and remarks!
Reading my post here (!), Robert, one of our developers , told me yesterday, that they have already discussed WebDAV internally a few months ago. Then Mark also suggested this way.
There even exists some extension https://www.mediawiki.org/wiki/Extension:WebDAV but as I understand it's in experimental status. But I've experimented with webdav on windows several years ago and the task was tricky because of various compatibility issues :)
It sounds promising, but there’s a lot to do and there are some risks. But maybe we should rethink this approach again.
I’m a bit skeptical about FTP and SVN. But that’s probably because I can’t visualize it at the moment…
Of course it will be sufficient if you'll make webdav work :) FTP will require a) creating some daemon that will reupload file uploaded via ftp to mediawiki via API. and b) finding a way to pass ftp link to excel in such way that excel will remember that's an ftp file. But just like that, with WebDAV you'll also need to find a way to correctly pass a link to msoffice.
I've just checked FTP - if you open excel as just "excel.exe ftp://host/path.xls" - it will really open that file for editing and when you click save it will be really saved to FTP location! :) also you can specify "ftp://host/path.xls" in File->Open dialog. But of course it doesn't work like that if you click on FTP link from a browser (even from IE) because the browser first downloads it to a temporary folder.