I guess it's correct to say there is no direct support for HL7, however...
JBoss ESB has support for Apache Camel: http://community.jboss.org/wiki/CamelGateway
and Camel has support for HL7: http://camel.apache.org/hl7.html
so that should provide a way to use HL7 through the ESB.
And for a different potential integration strategy you may want to look at this thread:http://community.jboss.org/thread/147132