Try configuring this on the datasource from which you obtain the connection:
I haven't tried it myself so not sure whether it's going to work. See this doc for the details http://community.jboss.org/wiki/configdatasources
That would work if I had 2 data sources (and 2 entity managers). The mysql replication driver works by using a master/slave setup in the same datasource.
Setting this to read-only would then tell the driver to only use the slaves (and never use the master), and hence defeats the purpose of using the replication driver.