So decorators are not supported for external beans no matter whether they are pushed or pulled.
Also, I found that when we have a "pushed" external bean with a producer method which is wrapped with an alternative Producer implementation (such as our ExisitingInstanceMethodProducer), I can't see anything in the CDI api to be able to figure out which bean manager the call came from. i.e. there does not seem to be anything in the ProcessProducer event or in the CreationContext passed in to Producer.produce(). This means it is impossible to have producer methods with injected parameters in these producer methods coming from external beans.
This works out of the box with the "pull" approach.