I think that you will have to be the original researcher for this kind of information. Many people love autochromes, but few are passionate enough to do the legwork.
The odds of finding the same thing that the Lumieres used is slim-to-none. You'll probably have to discover a new material that is available in modern times.
Your varnish will have to be very thin, and optically clear. Canada balsam comes to mind. Even a thin layer of gelatin might be suitable.
Good luck, and please share what you find.