gistfile1.txt

SAD.pm:
use SAD::Special;

class SAD {
  has $!nothing;
}


SAD/Special.pm:
role SAD::Special {
  has Bool $.special_thing = True;

  method apply_handles($att: Mu $pkg) {
    my $accessor = $att.name.substr(2);
    say "Apply to $accessor.";
    $!special_thing = False if $accessor ~~ 'other-thing';
  }
}

multi trait_mod:<is>(Attribute $a, :$special!) is export {
  $a does SAD::Special;
}


SAD/Thing.pm:
use SAD;
use SAD::Special;

class SAD::Thing {
  has $.thing is special;
  has $.other-thing is special;
}

This will re-export the trait mod so you only have to use SAD in Thing.pm

use SAD::Special;

sub EXPORT (|) {
    {
        '&trait_mod:<is>' => &trait_mod:<is>
    }
}

class SAD {
    has $!nothing;
}