Your scheme is sound in theory, but isn't a solution for Java under the Debian OS. It means that virtually all Java packages in Debian would sit in contrib. That's just not acceptable.
I see two options to solve this: 1) Strict interpretation of javaN-runtimeIndeed, probably no free JVM will be able to provide them at this point. However, it is still possible for packagers to make sure at least one free JVM works for their package, and depend on, say, kaffe | javaN-runtime
2) Looser interpretation of javaN-runtimeJVMs would be allowed to provide them, as long as they include some/most of the runtime. Then bug reports can be opened when a specific feature does not work. This bugs can be forwarded upstream to provide feedback about where work is most urgently needed. People willing to help free Java in Debian can also use these bugs reports to find tasks, and submit patches.
Daniel