Ok, for anybody else looking for this.
In the Properties object that you pass to PropertiesBasedEJBClientConfiguration set the following properties:
remote.connection.{connectionname}.connect.options.org.xnio.Options.SSL_JSSE_KEY_MANAGER_CLASSES
remote.connection.{connectionname}.connect.options.org.xnio.Options.SSL_JSSE_TRUST_MANAGER_CLASSES
to the Class names of your key and trust managers.
Cheers
Alan