I finally had time to do some searching, and this is what I found on my 2nd Gen, single drive, version of the My Cloud:
root@Storage2 /usr # find -name "halt"
root@Storage2 /usr # find -name "shutdown.sh"
Likewise, the only instances of "id_shutdown_td" or "dev_shutdown()" are in /usr/local/modules.
root@Storage2 / # grep -rnsl '/usr' -e "dev_shutdown"
root@Storage2 / # grep -rnsl '/usr' -e "id_shutdown_td"
The icing on the cake is that the /usr/local/modules portion of the filesystem is a loop-mounted squashfs filesystem. Viz.:
/dev/loop0 on /usr/local/modules type squashfs (loop)
1. It's a "virtual" filesystem segment loaded from a squashfs file located somewhere on the system.
2. It's not writable, so even if you COULD edit the file, you can't save it to see if it works!
The only way to play with this would be to find the actual squashfs filesystem file, copy it, un-pack it with 7Zip, edit it, and (somehow or other), put it back where you found it.
I haven't figured this out yet.
BTW, on MY My Cloud device, the "style.css" file I have doesn't even bear a remote resemblance to the one @Arquimaes found. I don't know what his system is, or whatever, but it does not resemble my system at all. . . .
I'm going to keep plugging at this, and I will report more when I know more.