Fire a user event down the channels when a handshake has occured.

fml/src/main/java/cpw/mods/fml/common/network/NetworkHandshakeEstablished.java

@@ -0,0 +1,24 @@
+package cpw.mods.fml.common.network;
+
+import cpw.mods.fml.common.network.handshake.NetworkDispatcher;
+import cpw.mods.fml.relauncher.Side;
+
+/**
+ * This message is sent through all channels affected by a currently occurring handshake. It is guaranteed to
+ * be able to send a custom payload packet, however, interaction with minecraft and world state is NOT assured
+ * as it is likely this is fired on a netty handler thread, not a world processing thread.
+ *
+ * If you wish to send an outbound message through your channel, bind the {@link FMLOutboundHandler#FML_MESSAGETARGET}
+ * property of your channel to the supplied dispatcher.
+ * @author cpw
+ *
+ */
+public class NetworkHandshakeEstablished {
+    public final NetworkDispatcher dispatcher;
+    public final Side side;
+    public NetworkHandshakeEstablished(NetworkDispatcher dispatcher, Side origin)
+    {
+        this.dispatcher = dispatcher;
+        this.side = origin;
+    }
+}

fml/src/main/java/cpw/mods/fml/common/network/NetworkRegistry.java

@@ -20,6 +20,7 @@ import io.netty.util.AttributeKey;
 
 import java.util.EnumMap;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Set;
 import org.apache.logging.log4j.Level;
 
@@ -34,6 +35,7 @@ import cpw.mods.fml.common.FMLLog;
 import cpw.mods.fml.common.Loader;
 import cpw.mods.fml.common.ModContainer;
 import cpw.mods.fml.common.discovery.ASMDataTable;
+import cpw.mods.fml.common.network.handshake.NetworkDispatcher;
 import cpw.mods.fml.common.network.internal.FMLProxyPacket;
 import cpw.mods.fml.common.network.internal.NetworkModHolder;
 import cpw.mods.fml.relauncher.Side;
@@ -423,4 +425,13 @@ public enum NetworkRegistry
     {
         return channels.get(side).keySet();
     }
+
+    public void fireNetworkHandshake(NetworkDispatcher networkDispatcher, Side origin)
+    {
+        NetworkHandshakeEstablished handshake = new NetworkHandshakeEstablished(networkDispatcher, origin);
+        for (Entry<String, FMLEmbeddedChannel> channel : channels.get(origin).entrySet())
+        {
+            channel.getValue().pipeline().fireUserEventTriggered(handshake);
+        }
+    }
 }

fml/src/main/java/cpw/mods/fml/common/network/handshake/FMLHandshakeServerState.java

@@ -58,6 +58,7 @@ enum FMLHandshakeServerState implements IHandshakeState<FMLHandshakeServerState>
                 ctx.writeAndFlush(new FMLHandshakeMessage.ModIdData(GameData.buildItemDataList()));
             }
             ctx.writeAndFlush(new FMLHandshakeMessage.HandshakeAck());
+            NetworkRegistry.INSTANCE.fireNetworkHandshake(ctx.channel().attr(NetworkDispatcher.FML_DISPATCHER).get(), Side.SERVER);
             return COMPLETE;
         }
     },