Posting for reference the proposed way on doing a visibility check on a mesh instance:
var MeshVisScript = pc.createScript('meshVisScript');
MeshVisScript.prototype.initialize = function() {
this.app.scene.layers.getLayerByName("World").onPostCull = (cameraIndex) => {
var entity = this.app.root.findByName("Box");
var meshInstance = entity.model.model.meshInstances[0];
console.log("visible: " + meshInstance.visibleThisFrame);
}
};