I used JBoss AS 7.1.2.Final. If you have not already you will have to download source and compile yourself. I recommend turning off the test cases when running the maven build.
1) modify the org.infinispan module.xml to have a dependency on the cache loaders and hotrod client.
<module name="org.infinispan.client.hotrod"/>
<module name="org.infinispan.cachestore.remote"/>
<module name="org.infinispan.cachestore.jdbc"/>
2) modify the web cache-container to use an invalidation cache backed by a remote-store. Similar to what Paul suggested with the jdbc-store.
<cache-container name="web" aliases="standard-session-cache" default-cache="remote" module="org.jboss.as.clustering.web.infinispan">
<transport lock-timeout="60000"/>
<invalidation-cache name="remote" mode="ASYNC" batching="true">
<remote-store passivation="false" purge="false" shared="true">
<remote-server outbound-socket-binding="session-datagrid"/>
</remote-store>
</invalidation-cache>
</cache-container>
3) Add an outbound-socket-binding to the socket-binding-group your server is using. This is the host and port of your remote infinispan cache.
<outbound-socket-binding name="session-datagrid">
<remote-destination host="localhost" port="11222"/>
</outbound-socket-binding>
4) Start an infinispan cache with the hotrod connector.
jboss\infinispan-5.1.5.FINAL\bin>startServer.bat -r hotrod
This got me up and running with a simple two node cluster fronted with mod_cluster. Still lots more testing and optimizations required but please share your experience.