This answer comes late, but here is the solution for TomCat. Hope it will help someone.
You have to add a ResourceLink in your Context.xml, in order to let Hibernate using the global resource, witch is in read only mode.
Code:
<ResourceLink name="jdbc/ora" global="jdbc/ora"
type="javax.sql.DataSource"/>
Here is the explaination from the book "
Professional Apache Tomcat 5"
"The <ResourceLink> element refers to a previously configured JNDI resource (typically in the
<GlobalNamingResource> sub-element associated with a server), making these resources available to
all <Service>, <Host>, and <Context> components. This enables resources to be defined and shared
across servers or globally."