Hah, I have an answer. It's nothing to do with Extenalizable, or Scala's abbreviated constructors (or inner classes, or name-mangling). It's a clash of identifiers fooling the Scala compiler. Rename all identifiers in the base class (or the subclass). You may want to refer to the answers to this question,
For anyone interested, with this adjustment, the above is working code.
Bear in mind that all (known to me) Java serialization techniques are available, or can be made available, in Scala,
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