Hello @ideasman42,
Within Blender the closest thing non-technical users are coming to for full UI-customization is the Pie Menu Editor plugin.
Like the new Top bar in Blender, a dockable menu which is globally usable
would be sufficient where you can drag and drop Blender icons into.
Within ZBrush, you are not destroying or hiding the original menus/buttons but instead you are “linking” them into another (hidden) bar which feels like a copy pasting procedure.