Except, oops! This won't boot. We need to put GRUB on it.
Well here's where I'm stuck for tonight. I don't know exactly what's
needed; maybe either the -b flag, or --grub2-mbr, or some combination.
The man page is a little bit overwhelming (I mean, xorrisofs is clearly
featureful, credit there):
https://www.gnu.org/software/xorriso/man_1_xorrisofs.html
But how do I generate the right GRUB stuff to put there? Can I pull it
off the USB image? Generate it separately?
This web page is very long but appears to have the appropriate info (and
unfortunately requires running arbitrary javascript to even render):
http://lukeluo.blogspot.com/2013/06/grub-how-to-2-make-boot-able-iso-with.html
... so I guess the next steps are following roughly what's described at
the bottom of the page?
I wonder how other distros do this step.