The bike in question in my OP isn't currently using a crown-mounted stop. But...I know it's drilled for one becasue I recall looking at the hole and thinking that I could mount a spare side-pull caliper on it (but obviously didn't cause it'd leave the canti lugs weirdly exposed). Although the smaller pads have mostly eliminated the shudder...I've told the kid that I want to properly correct it...told him to bring the bike next time he's home from school.
Still the cause/correction is baffling to me. I just don't see how moving the cable stop from the steerer to the fork crown eliminates the shudder.
Dan