From what you mentioned, I assume that other the client side jdbc there is no Java dependency from Server side. What I mean is I assume there is no jdk/java dependency for Postgres server to work. Request your input.
If you want to use PgJDBC, then yes, PgJDBC works on OpenJDK. You do not need to use pgjdbc 9.4 with PostgreSQL 9.4, you may use the latest version.
If you want to use pl/java, then I don't know if it's been tested with postgres-bdr 9.4. But it should work fine if it works with normal community postgres 9.4.