In my particular use case, the classes are from some internal legacy project. I think your suggestion

...or if it cannot be shared then we can write couple of macros for generation of Scala classes for Java POJOs with bidirectional conversion functions for them.

would be ideal: having some way to add a serializer from bidirectional conversion functions would be great. Doesn't even have to be performant (or safe w.r.t. security). Would probably make this (fantastic!) project much wider applicable!