Since you only allow one implementation at the moment, the serialization service actually choses between the available implementation and provides a serializer?

Correct.

Or have you disabled the ability to chose a serializer?

No. Now you actually can. Before it was hard wired to Avro (even w/ a comment it should be configurable)

I guess for this case, if the user customizes the CL service, it would provide it and if none is provided we start our own impl.

Something alone these lines