And that's the tough part.
If you do what Matth said, you introduce an exploit and a balance issue as well. If players notice it, that guardianed wingmen can be used for anything. It may not cause much of a problem in some missions (e.g. escort missions), but for assault missions, this wingmen could be used (exploited) to do anything. So unless you have all your wingmen-sent messages early in the mission, this method isn't as good as it seems.
Now, you can script the messages so that if X dies, then Y will say the lines, and if Y dies, a third wingmen will say the line. <Any wingmen> is straightforward, but the wingmen closest to Alpha 1 in designation will send the messages. If Alpha 2 is alive, he will send all <any message> lines. I prefer this solution for individual messages that any wingmen can send, for lines such as "We're getting overwhelmed." By individual message, I meant messages that are not sent as part of a conversation. However, as soon as you want a dialog to take place between two wingmen, things get more complex.
Also no one will notice how much effort you placed in getting your lines right, no matter how complex the "send-message" events are. One malevolent hate-mongerer will always point out a single flaw in the mission, go "this sucks you noob" all over the place, implying that the mission is badly constructed, you're a noob, and your campaign must disappear in the trashcan. At that point, it's already too late to say anything, everyone will view your work as a piece of garbage.
Long story short, unless a message MUST be sent by a wingman/pilot, have Command send that message.