If you look further down in jboss_xmbean_1_2.dtd, you'll see this injection is meant specifically for MBeanServer, MBeanInfo and ObjectName.
"Injection" of attribute values is usually done in jboss-service.xml
Sorry that this is such a late response...
I was actually trying to use the injection mechanism for MBeanServer, ObjectName etc - foo and bar are just for illustration ;)
But I couldn't get ...injection id="MBeanServer" setMethod="setMBeanServer... accepted by the XML validation phase of deployment.
I'm no DTD expert, which prompted me to ask the question - it looks to my untutored eye that the DTD is missing some information. Has anyone actually got one of these injections to work?
That was actually a bug, solved here: http://jira.jboss.com/jira/browse/JBAS-2501.
i.e. you couldn't use 'injection' with a standalone xmbean descriptor. The xml parser validation phase was failing. It worked ok with an embeded xmbean descriptor.