Clearly we can associate a unique (usually integer in our case) class ID / discriminator to each persistent class.

Given that this class ID is always unique across all classes within our system I have found a use case where using that concise unique integer would be beneficial/optimal for use outside of persistence (e.g. concisely identifying classes across microservice linked components without having to supply a FQCN) - the trouble is, it's embedded in the metadata (.jdo) file at build time and I haven't been able to work out a convenient way of retrieving it at runtime.

Does the JDO API or any DN extension support, given a persistent class i.e. Class<>  (not an actual persistent object instance), retrieving the discriminator (ClassID) associated with that class?

I realize that the discriminator is not only optional but it also supports a variety of types, not just integer but perhaps there is an internal discriminator "object" that holds this info that we could extract from.

The discriminator is clearly in the retrieval SQL when you query for objects, but is only used to decide which type of class to instantiate.
It would be part of the metadata for the class, so you could use the JDO MetaData API to extract the discriminator value for a specific class

pmf.getMetadata(MyClass.class.getName()).getInheritanceMetadata().getDiscriminatorMetadata().getValue();