Customize Cluster Symbol for a Feature Layer for JS API 4.x

[AlanScandrett]

When clustering a Feature Layer, can we have a different icon for the cluster than the features themselves?

 Esri Leaflet seems to accomplish exactly what I am describing below (see link and gif): 

https://esri.github.io/esri-leaflet/examples/styling-clusters.html 

Have one symbol for the cluster:

   

That is different from the feature symbology (PictureMarkerSymbols):

when the symbology for the 

Is the above possible with the JS API 4.X using the featureReduction capability or another means? I have exhausted the documentation and scoured other resources online, the only direction of hope seems to be using the CIMSymbol to work around it somehow.

Layer:

Feature Layer (points) using a Unique Value renderer where each value as a custom PNG icon. 

[bmanghi]

Apparently there's an undocumented symbol property in FeatureReductionCluster - https://community.esri.com/t5/arcgis-api-for-javascript-ideas/arcgis-javascript-4-cluster-renderer/i...