I would not do so if it was *only* for a "dead ball personal foul that would have been a horse collar if it was still in the field of play". That's way too specific. What if it was targeting? Or just an unnecessary late hit? It would have to be all dead ball personal fouls, or none.
I'm generally not for automatic first downs. I know that's what they do in the NFL, but their goal is to increase offense, increase exciting plays, increase scoring and keep the fans interested. That's not (or shouldn't be) the goal of high school football and I don't think we should emulate those sorts of rules just because it's As Seen On TV. I certainly don't want to end up with their rule where everything is an AFD -- except for 10 situations where 3 are offsides, 3 are illegal substitutions, 2 are delay of game and 2 are scrimmage kick fouls (running into the kicker and illegal formation), so really only 4.
It would be an interesting (and tedious) study to see how many major (15 yard) fouls do NOT result in a first down by yardage as a whole. My guess is that there's a vanishingly small percentage of major defensive fouls on plays with more than 15 yards to the LTG, and a significantly larger (but still small) percentage that are in Goal-to-Go situations.
I don't like this as a weird carve out, but I wouldn't be as horribly opposed to AFD on major penalties when the ball is snapped inside the 20 (or so...). If the foul happened at the 40, the 15 yards would normally grant a 1st down, but since it's half the distance and therefore less than 10 yards, it may not result in a first down.