Made another attempt at using the script with a SquashFS image. No bueno (didn't work).
Again, the script properly resized the partition but not the filesystem. No errors reported by the script.
I was able to resize it manually by booting the board off a different device (microSD in this case) and using the tools onboard to fsck and resize.
losetup -o XXYYZZ /dev/loop0 /dev/mmcblk1p2
fsck.f2fs /dev/loop0
resizefs /dev/loop0
Where XXYYZZ is the offset reported earlier by running losetup on /dev/loop0 when OpenWrt was running off it.
Then I did this for SquashFS: