Installing the installer

[Richard Owlett <rowlett@cloud85.net>]

Yes, I know the subject line looks strange. It's intentional.
I wish to do something not quite normal ;/

I wish to "install" to an arbitrary ext2/ext4 partition in such a manner 
that:
  1. Grub2 will recognize it as a legit OS.
  2. if the partition is on a USB flash drive it will boot normally
     on suitable hardware.
  3. all directories and files shall be modifiable.

I had done
    xorriso -osirrox on:auto_chmod_on -overwrite nondir \
      -indev "/home/richard/Downloads/debian-9.1.0-i386-netinst.iso" \
      -extract / /media/richard/netinst1

There were no error messages but Grub2's "update-grub" would not 
recognize it nor would my hardware boot it.

I suspect there something of the boot process that I don't grok.
Guidance please.