I've confirmed that changing the following setting in the attached test will work:
hibernate.transaction.jta.platform=org.hibernate.engine.transaction.jta.platform.internal.WebSphereJtaPlatform

WebSphereJtaPlatform basically does the same thing that Christian's custom JtaPlatform does.

Using the short name should also work, but I haven't tried it yet:
hibernate.transaction.jta.platform=WebSphere