It is not possible to change the diagnostics multicast and port after settings the system property -Djboss.jgroups.diagnostics_addr= -Djboss.jgroups.diagnostics_port=

They do appear in the boot.log as expected, but the output from netstat -ng and jgroups trace logs still shows the default host and port 224.0.75.75 being used.
This occurs because the address and port are hard-coded in $JBOSS_HOME/$PROFILE/conf/bindingservice.beans/META-INF/bindings-jboss-beans.xml

The following section in $JBOSS_HOME/$PROFILE/conf/bindingservice.beans/META-INF/bindings-jboss-beans.xml

<bean class="org.jboss.services.binding.ServiceBindingMetadata">
<property name="serviceName">JGroups</property>
<property name="bindingName">diagnostics</property>
<property name="hostName">224.0.75.75</property>
<property name="port">7500</property>

needs to be changed to

<bean class="org.jboss.services.binding.ServiceBindingMetadata">
<property name="serviceName">JGroups</property>
<property name="bindingName">diagnostics</property>
<property name="hostName">${jboss.jgroups.diagnostics_addr:224.0.75.75}</property>
<property name="port">${jboss.jgroups.diagnostics_port:7500}</property>