If you're going to preserve a signature on the finish, you need to prep the finish first, that means sanding - otherwise a protective coating will not stick to the smooth, buffed surface of the existing finish. It's not easy , and if you're not experienced, I wouldn't recommend experimenting on your Alembic.
Once your finish is prepped, you'll need to find a suitable marker. Sharpies are common, but you need to check that it won't react with whatever you're going to protect the signature with. For instance, Krylon spray paint likes to melt away Sharpies, so test your marker and your cover paint first.
Wow, getting the inside of the backplate signed is sounding better and better all the time!