I'm not trying to convince you to keep the effect of the Slayer theme, but...
... doing a small amount of HP damage on a failed attack roll makes perfect sense from an AD&D perspective --is that old-school enough?-- and is congruent with the AD&D's conception of HP.
Because in AD&D, rounds a minute long and represent a whole sequence of strikes, dodges, feints, grazing blows, etc. The to-hit roll is really a "to-damage" roll, the one hit out of many that actually caused damage (and remember, damage ain't all physical, not by a long shot).
Even with these new-fangled 6 second rounds, a single attack roll is going to be more than a single swing of a weapon.
Considered from this perspective, a Reaper fighter is just particularly good at wearing opponents down, chipped away at their defenses, bruising them, whatever. They're endurance fighters -- like Rocky Balboa.
It's not that they can hit when they miss, it's just means engaging them in melee inevitably wear you out.
This for me, coupled with what HP represent - not merely physical damage capacity, but luck, ability to "roll with the blow" to lessen the effect, mental toughness in staying focused during adversity, etc.
So an attack misses - how much of a stretch is it to think that some of the target's "luck" is used up? Or some of their focus is lost as they worry about that giant 2-handed axe that just missed them?
If HP solely represented ability to take physical damage, I'd agree with everyone that the slayer ability is lame.
However, because HP represents so much more than that, I would certainly keep the ability.