Well I can't just do it for send-message-list. It would have to be done for at least send-message too and possibly other message SEXPs. I doubt you're complaining about that though.

Looking at the code this is perfectly legal for any of the message SEXPs.
send-message
-Beta
-high
-Message Name
However both FRED and the mission parse code won't let you do that. The code is still there and uncommented (as evidenced by the fact that I thought it was live and actually altered it when I added the radio silence code) but there is no way to reach it.
The code does have this comment in it.
// message from a wing
// this will be an invalid case soon
// Int3();
// choose wing leader to speak for wing (hence "1" at end of ship_get_random_ship_in_wing)
It looks like

planned to add exactly what you describe but forgot about it halfway through or decided against it. Since they never made it work I've got
carte blanche to make it work sensibly rather than the stupid way they have it working in their half-finished code at the moment (It would only ever pick the wing leader and if he wasn't available it would pick any wingman from any wing).

So I'll simply make it do this, you can add any wing, it will first attempt to pick a ship at random from that wing who hasn't been radio-silenced. If it can't find one it will then pick any wingman in that wing. If that fails it sends high priority messages from command or sends nothing at all.
I suspect that's exactly what most FREDders would expect it to do.