Unless I misunderstood the post, that is the way it is now. For example with one of your 4v4 missions in a custom arena with two squad mates, 3 live players would be assigned to either side and the other 5 would be AI.
I think he means the ad hoc missions created in the Melee.
[Edit]
So like currently (if I'm remember right) if you created an ad hoc Melee mission with 4 bomber but only one player joins, it only spawns one set. I think he is saying fill the other slots not taken by players with more AI bombers.
That would be intriguing and would promote more missions, and could expand off the current Melee ad hoc mission def without adding permanent ongoing AI missions.
You'd have to put some reasonable constraints on it.

Like maybe X limit on planes in any one mission and Y number of mission planes alive at any one time to avoid AI spamming.

But as you see, there would be so much inherent bias against that that there would be revolution.