Duplicating the skull and sculpting on the duplicate is the closest to what I’m talking about. But a moment’s reflection should be be enough to see the drawback in the workflow you’re describing. The duplicate you sculpt on is free to go below that original skull. Any time you smooth and any time you do something like clay with “sub” on, you’re potentially carving below that base.
I’m simply describing a sculpting method that’s closer to the real world activity anthropologists do when they have a skeleton and put flesh on it. It’s just an example of a broader category of sculpting that currently no program seems to let you do. Obviously we’re all aware that you can sculpt a head without doing the skull first. That’s what innumerable internet tutorials show. But whatever.