Later in the day, I saw that I could solve this by defining the superclass as default implementation, and the subclass would be instantiated when necessary:

services.DeclareService(null, 
                       [typeof(CommonWriter)],
                        DeclarationPolicies.IsDefaultImplementation);
services.DeclareServicesFor<CustomWriter>()

the CommonWriter probably overrun all implementations because it was the first to be registered.