Hey Stuart,
There was a recent Sandcastle example here that demonstrated how to scale a 3D Tileset vertically while accounting for when the origin is far from what’s being scaled: How to change the scale for box defined tilesets?.
But I’m not sure if that solution readily applies in your use case. Perhaps this could be something built-in to CesiumJS to make this transformation easier. Can you tell me more about your use case? Are you mainly interested in vertically scaling point clouds? Or is this tileset just created for debug purposes? How are your 3D Tiles generated? Is the goal to be able to see changes in elevation more easily?