I'm not familiar with Deltaspike. My feeling is the entity manager should be scoped to a step, since other steps in the same job may use other resources. For a partitioned step execution, I think the same entity manager can still be safely used by multiple partitions of the same step concurrently, though I haven't tried that.
Thanks for your interest. Here at JBeret we always welcome and value developer and user contribution. We could potentially house that somewhere under GitHub - jberet/jsr352: Implementation of JSR 352 as an extra module. Let us know when it's ready for a review.