I am not aware of any tool that will let you do that online. One can operate in the old style of sharing documents and comments, and e.g. check in/out versions into Git (using Git inefficiently, but so what). There are online sharing tools for licenses and rights to edit, http://www.iceni.com/infix.htm
but that is not what we want. The tool has features specially for translators, and might be interesting from that point of view.
The best online solution for us seems to be to continue to use wiki, which we need anyway, and have scripts to ease conversion to pdf from that source. We could use programmer time to develop a custom script to do that using http://stackoverflow.com/questions/6965430/how-to-create-pdf-of-dynamic-contain-using-php-script
so that we are not hand editing image maps. A custom script will give better results than the easier HTML to pdf conversion.