JBoss has an HA version of the pooled invoker that is not in the cluser-service.xml or in the formal documentation.

To use it add

   <mbean code="org.jboss.invocation.pooled.server.PooledInvokerHA"
      name="jboss:service=invoker,type=pooledha">
      <attribute name="NumAcceptThreads">1</attribute>
      <attribute name="MaxPoolSize">300</attribute>
      <attribute name="ClientMaxPoolSize">300</attribute>
      <attribute name="SocketTimeout">60000</attribute>
      <attribute name="ServerBindAddress">${jboss.bind.address}</attribute>
      <attribute name="ServerBindPort">4446</attribute>
      <attribute name="ClientConnectAddress">${jboss.bind.address}</attribute>
      <attribute name="ClientConnectPort">0</attribute>
      <attribute name="EnableTcpNoDelay">false</attribute>

      <depends optional-attribute-name="TransactionManagerService">jboss:service=TransactionManager</depends>
   </mbean>

to server/all/deploy/cluster-service.xml

Then edit your server/all/conf/standardjboss.xml and essentially replace all jboss:service=invoker,type=jrmpha strings with jboss:service=invoker,type=pooledha or create and override your own proxy bindings up to or on a per-bean basis in the jboss.xml.

Generally speaking, most folks think NumAcceptThreads should be one (this is the number of threads blocked on ServerSocket.accept()).  MaxPoolSize+tcp backlog is a hard limit so set it to a reasonble number for your expected traffic.