The inclusion of system properties in cluster configuration is a very nice feature.

It is my understanding that to make a unique cluster(when using UDP), one must specify all three of "mcast_addr", "mcast_port", and "jboss.partition.name:DefaultPartition".

If this is the case, then would it make sense to include "mcast_port" system property and a related run.sh argument ?

I've changed my cluster-service.xml to include this, and it does work(also nice that it works for this attribute as well)
<UDP
mcast_addr="${jboss.partition.udpGroup:228.1.2.3}"
mcast_port="${jboss.partition.mcastPort:45566}"
ip_ttl="${jgroups.mcast.ip_ttl:8}"
ip_mcast="${jboss.cluster.mcast.enabled:true}"
mcast_recv_buf_size="2000000" mcast_send_buf_size="640000"
ucast_recv_buf_size="2000000" ucast_send_buf_size="640000"
loopback="${jboss.cluster.use.loopback:false}"/>

run.sh -c all -u224.1.1.39 -Djboss.partition.mcastPort=45577

This is probably only an issue if running more than one node on a machine