The best way depends on how you would/could contribute, i.e. time to time or maybe more regularly (up to join me).
You are right, the codebase is still a work in progress and I am still practicing major refactorings. So, it is hard to contribute to the core unless we work together closely.
There are some tasks that don't depend much on the engine and you could contribute working on them. For example, id generators, database schema generation, enhancer.
Does it look interesting? Do you have your own ideas?
Let's discuss them.
And one of the most appreciated ways to contribute is to actually use the framework and give feedbacks ;)
What's the package for schema generation? I'd like to experiment with this. Is it org.jboss.persistence.schema? It looks like you (or someone) is making sure it works the same as the older CMP engine.