Just an idea...
Use an "intermediate sheet" of something like mylar. Sign that with black opaque ink - you may have to try several to find one that sticks.
Then put the intermediate sheet between the glass and the negative.
The drawback is that it's two more surfaces where dust can collect. The advantage is that you can write the right way around, and don't have to worry about mirror writing as you would if you signed the backside of the glass. The sheet would also be easy to change, so you could write the title of the picture too - just like in "old tymes".