How to get syslinux to install to fat32 backup boot sector

How do you get syslinux to install to fat32, and have it write the backup boot sector. It only writes to the main boot sector, and then fsck.fat complains. You can get fsck.fat to fix it, but this requires running it in interactive mode, and hence is not possible from a script.

/tmp # fallocate -l 50m test_image
/tmp # mkfs.fat -F32 test_image   
mkfs.fat 4.1 (2017-01-24)
/tmp # syslinux --directory syslinux --install test_image

/tmp # fsck.vfat test -a
fsck.fat 4.1 (2017-01-24)
There are differences between boot sector and its backup.
This is mostly harmless. Differences: (offset:original/backup)
  3:53/6d, 4:59/6b, 5:53/66, 6:4c/73, 7:49/2e, 8:4e/66, 9:55/61, 10:58/74
  , 90:fa/0e, 91:fc/1f, 92:31/be, 93:c9/77, 94:8e/7c, 95:d1/ac, 96:bc/22
  , 97:76/c0, 98:7b/74, 99:52/0b, 100:06/56, 101:57/b4, 102:1e/0e, 103:56/bb
  , 104:8e/07, 105:c1/00, 106:b1/cd, 107:26/10, 108:bf/5e, 109:78/eb
------------ SNIP ---------------------------------------------------------
  , 484:0d/00, 485:0a/00, 504:fe/00, 505:02/00, 506:b2/00, 507:3e/00
  , 508:18/00, 509:37/00
  Not automatically fixing this.
test: 2 files, 353/100792 clusters

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Author: Gary van der Merwe