I bought a roll of white glossy magnetic tape which I use to label the channel faders. I use a Sharpie pen to write on the tape, it is permanent but can be cleaned off with alcohol. I have several pre-cut strips sticking to the underside of the mixer and when I need to label a new scene there is a fresh one ready to use.
Existing scenes are preserved in this way too. It would be a simple matter to do the same for the custom layer. In a gig where many different layers of scenes are used it would not be so useful though.