Funny how the answer is obvious once you ask.

I noticed the "Data Offset" was 2048 sectors.

On the newly-created md, it was 16 sectors, I think because it was a
device mapper overlay and not a real block device.

I found this page:
http://www.thomas-krenn.com/en/wiki/Linux_Software_RAID

Got the mdadm development version that allows setting data offset.

  724  parallel truncate -s4000G overlay-{/} ::: $DEVICES
  725  parallel 'size=$(blockdev --getsize {}); loop=$(losetup -f --show -- overlay-{/}); echo 0 $size snapshot {} $loop P 8 | dmsetup create {/}' ::: $DEVICES
  726  cat /proc/mdstat 
  727  mdadm -S /dev/md127
  728  ~/src/mdadm-40c9a66/mdadm --create  /dev/md127  --data-offset=1024  -v -l 0 -n 4 $OVERLAYS

That fixed it up (data offset is in k, not sectors).

Cheers.
-- 
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