Yes, you can define your own ellipsoid and pass it to the Viewer. Here is a code example:
var ellipsoid = new Cesium.Ellipsoid(1, 1, 1);
var globe = new Cesium.Globe(ellipsoid);
var viewer = new Cesium.Viewer(‘cesiumContainer’, {
globe: globe
});
``
Most functions in Cesium either get the ellipsoid from the scene, or have a parameter to pass in your ellipsoid to override the default WGS84 ellipsoid.
Please let us know if you find any bugs! Your custom globe should be used by our code in most places, but since very few users use a custom globe, you might find some things we overlooked and we will be happy to try to fix it.