In the nonspoiler FAQ thread:
Is a player who has been knocked out but has not yet had a chance to respawn permitted to use a missile, or are you required to have at least one unit on the board to do this? For example, I am third in a three player game. Player 1 wipes me off the board, but there are plenty of places to respawn. Can I use my missile during player 2's turn to modify their combat with player 1, or must I wait until I have had my turn and respawned some units onto my new starting location before that missile is once again available to me?
Hadn't thought of that. Good question.
I would say, yes. A person who is knocked out (but not eliminated) can use missiles to affect combat. Technically, he/she is still playing the game at that point.
And what a delicious way to exact revenge. Or justice. Or vengeance. Whatever you want to call it.