It's been a month since you asked but short answer to the question is yes, it works, Bitronix is not required. I'm using WAS7 with Oracle 10G.
TransactionManager TM =new WebSphereTransactionManagerLookup().getTransactionManager(new
and then in your persistence.xml:
<property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.WebSphereTransactionManagerLookup"