Ended up drilling it. At Home Despot, 6mm metric bolts didn't even come as long as the supplied one. 1/4" seemed imperceptibly close (to within the measurement accuracy of my $3 calipers) to 6mm, so I got a 1/4" bolt and nut. Turns out 1/4" is 6.35mm, and that's large enough so that I had to drill the hole in the hanger for starters, and I would have to expand both holes in the fork as well. So I decided I might as well just drill the fork. As assured above, it turned out to be quite easy. Fortunately, I had an individually-purchsed 5/16" bit from some other project (much better quality than my kit of smaller bits), and it drilled through in about 30sec. When I tried to hook it up, turns out my drilling was not straight, but angled downwards. So back in there with the drill, and wiggle it around, bingo, I'm good to go.
AND, the fork-mount brake hanger did the job as promised, brake shudder is GONE.