Hi. Even though OpenJPA 2.1.x is built with Java 6, it should still run okay with Java 5. You won't have access to the MetaModel annotation processor (code generation), but that wouldn't be new for your environment. I am in the process of updating this old article  to explain how to use OpenJPA 2.x in WebSphere v6.1 environment (non-managed, of course) and I have not had any problems running the samples and tests in this environment. The updated article should be posted soon on Developer Works.
I'm not trying to justify not fixing this in OpenJPA 2.0.x, but I thought I would let you know that I think the OpenJPA 2.1.x driver would work for you as well.