I thought this was the already the case, but apparently it isn't yet.
Users should not need to specify a value for hibernate.cache.region.factory_class in their persistence.xml.
If undefined, the jpa subsystem should default the hibernate.cache.region.factory_class to "org.hibernate.cache.infinispan.JndiInfinispanRegionFactory". Likewise, the requisite hibernate.cache.infinispan.cachemanager property should default to "java:jboss/infinispan/hibernate".