how I could anchor the pad to the output?
That was my point, I don't think you can. They didn't link the volume to the saved output:
The volume level settings are not stored with the output presets, as switching the presets could result in dangerous volume jumps.
What I believe was overlooked was the - 12db pad IS stored in the presets so switching between one preset -12 and one with it off could result in big jumps.