[41Sqn_Banks]

This new coop script doesn't use the mission menu, instead the commands are typed into the chat console. Thus the dummy aircraft are no longer needed.

Steps:
1) You must add the following line to your conf.ini or confs.ini (if you want to use the dedicated server):

[rts]
scriptAppDomain = 0

2) Create a new mission or use a existing single player mission
3) Add the following script to the mission:

using System;
using System.Collections.Generic;
using System.Text;

using maddox.game;
using maddox.game.world;

//$reference parts/core/gamePlay.dll

public class Mission : AMission
{
    public override void OnBattleInit()
    {
        base.OnBattleInit();

        if (GamePlay is GameDef)
        {
            (GamePlay as GameDef).EventChat += new GameDef.Chat(Mission_EventChat);
        }
    }

    void Mission_EventChat(IPlayer from, string msg)
    {
        if (msg.StartsWith("!help"))
        {
            Chat("Commands: !aircraft, !select#", from);
        }
        if (msg.StartsWith("!aircraft") || msg.StartsWith("!select"))
        {
            List<Tuple<AiAircraft, int>> aircraftPlaces = new List<Tuple<AiAircraft, int>>();
            if (GamePlay.gpArmies() != null && GamePlay.gpArmies().Length > 0)
            {
                foreach (int army in GamePlay.gpArmies())
                {
                    if (GamePlay.gpAirGroups(army) != null && GamePlay.gpAirGroups(army).Length > 0)
                    {
                        foreach (AiAirGroup airGroup in GamePlay.gpAirGroups(army))
                        {
                            if (airGroup.GetItems() != null && airGroup.GetItems().Length > 0)
                            {
                                foreach (AiActor actor in airGroup.GetItems())
                                {
                                    if (actor is AiAircraft)
                                    {
                                        AiAircraft Aircraft = actor as AiAircraft;
                                        for (int place = 0; place < Aircraft.Places(); place++)
                                        {
                                            aircraftPlaces.Add(new Tuple<AiAircraft, int>(Aircraft, place));
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }

            if (msg.StartsWith("!aircraft"))
            {
                int i = 0;
                foreach (Tuple<AiAircraft, int> aircraftPlace in aircraftPlaces)
                {
                    string playerName = "";
                    Player player = aircraftPlace.Item1.Player(aircraftPlace.Item2);
                    if (player != null)
                    {
                        playerName = " " + player.Name();
                    }
                    Chat("#" + i + ": " + aircraftPlace.Item1.Name() + " " + aircraftPlace.Item1.TypedName() + " " + aircraftPlace.Item1.CrewFunctionPlace(aircraftPlace.Item2) + " " + playerName, from);
                    i++;
                }
            }
            else if (msg.StartsWith("!select"))
            {
                msg = msg.Replace("!select", "");

                int i = -1;
                if (int.TryParse(msg, out i) && i < aircraftPlaces.Count)
                {
                    Tuple<AiAircraft, int> aircraftPlace = aircraftPlaces[i];
                    if (aircraftPlace.Item1.Player(aircraftPlace.Item2) == null)
                    {
                        from.PlaceEnter(aircraftPlace.Item1, aircraftPlace.Item2);
                    }
                    else
                    {
                        Chat("Place occupied.", from);
                    }
                }
                else
                {
                    Chat("Please enter a valid aircraft number, e.g. !select0, !select1, !select2, ...", from);
                }
            }
        }
    }

    public void Chat(string line, IPlayer to)
    {
        if (GamePlay is GameDef)
        {
            (GamePlay as GameDef).gameInterface.CmdExec("chat " + line + " TO " + to.Name());
        }
    }
}

4) Host the mission. Clients have access to several commands by typing the command into the chat:

!help - this will show the available commands
!aircraft - this will show a list of all available aircraft starting with the id of the aircraft
!select# - this will place the player into the aircraft, replace # with the id of the aircraft, e.g. !select0 will place you into the first aircraft and so on.