Display a fake pumkin helmet for all users. DEPRECIATED: Use https://gist.github.com/aadnk/11323369 instead.

FakePumpinHelmets.java

package com.comphenix.example;

import java.lang.reflect.InvocationTargetException;

import org.bukkit.Material;
import org.bukkit.entity.Entity;
import org.bukkit.entity.LivingEntity;
import org.bukkit.entity.Player;
import org.bukkit.inventory.ItemStack;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;

public class FakePumpinHelmets extends JavaPlugin {
    // http://wiki.vg/Protocol#Entity_Equipment_.280x05.29
    public static final int SLOT_HELD = 0;
    public static final int SLOT_BOOTS = 1;
    public static final int SLOT_LEGGINGS = 2;
    public static final int SLOT_CHESTPLATE = 3;
    public static final int SLOT_HELMET = 4;
    
    @Override
    public void onEnable() {
        ProtocolLibrary.getProtocolManager().addPacketListener(
          new PacketAdapter(this, ConnectionSide.SERVER_SIDE, 
           Packets.Server.ENTITY_EQUIPMENT, Packets.Server.NAMED_ENTITY_SPAWN) {
            @Override
            public void onPacketSending(PacketEvent event) {
                PacketContainer packet = event.getPacket();
                
                // The entity that is being displayed on the player's screen
                Entity visibleEntity = packet.getEntityModifier(event).read(0);
                // The player that is receiving the armor information
                Player observingPlayer = event.getPlayer();
                
                switch (event.getPacketID()) {
                    case Packets.Server.ENTITY_EQUIPMENT :
                        // Determine if this is a helmet slot
                        if (packet.getIntegers().read(1) == SLOT_HELMET) {
                            packet.getItemModifier().write(0, new ItemStack(Material.PUMPKIN, 1));
                        }
                        break;
    
                    // Entity equipment is not sent when the helm slot is empty, so we have to help it along
                    case Packets.Server.NAMED_ENTITY_SPAWN:
                        // Update the head slot too
                        LivingEntity living = (LivingEntity) visibleEntity;
                        updateEquipmentSlot(observingPlayer, living, SLOT_HELMET, living.getEquipment().getHelmet());
                        break;
                    default:
                        throw new IllegalArgumentException("Unknown packet ID:" + event.getPacketID());
                }
            }
        });
    }
    
    private void updateEquipmentSlot(final Player observer, LivingEntity entity, int slot, ItemStack content) {
        final PacketContainer equipmentPacket = new PacketContainer(Packets.Server.ENTITY_EQUIPMENT);
        equipmentPacket.getIntegers().
            write(0, entity.getEntityId()).
            write(1, slot);
        equipmentPacket.getItemModifier().write(0, content);
        
        // We have to send the packet AFTer named entity spawn has been sent
        getServer().getScheduler().scheduleSyncDelayedTask(this, new Runnable() {
            @Override
            public void run() {
                try {
                    ProtocolLibrary.getProtocolManager().sendServerPacket(observer, equipmentPacket);
                } catch (InvocationTargetException e) {
                    throw new RuntimeException("Unable to update slot.", e);
                }
            }
        });
    }
}

@SupaHam: You're supposed to call updateEquipmentSlot() with the slot you want to modify.

But I think I'll just rewrite this example with a better API.