While flagging the illegal kick and letting the play proceed "feels" like the right thing to do, I suspect it's actually the wrong ruling.
4-1-1 tells us that the ball must be put in play by a kickoff.
2-23-5 tells us that a kickoff is a free kick.
2-23-3 tells us "A free kick is any legal kick which puts the ball in play to start a free-kick down."
Since the kick isn't legal, the ball is never put into play - i.e. it remains dead, just as if there were encroachment.
Can we flag this? I see no reason why not - nothing in the rules specifies that the illegal kicking rules only apply to live balls. But we should shut this down immediately, penalize the 15 yards, and re-kick.
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