It was painful but I got Ubuntu ISO working with

  loopback loop (hd0,gpt1)/PATH/TO/UBUNTU.iso
  set root=(loop)
  configfile /boot/grub/loopback.cfg

which in turn seems to load the kernel via

  linux  /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed iso-scan/filename=/PATH/TO/UBUNTU.iso
  initrd /casper/initrd

if you just make sure your path doesn't have spaces. (I forget if you also need to pass boot=casper or not.)

You'll have to detect the disk though. It's possible to automate it somewhat, but not trivial (at least if you don't know how). You'll want to use search. https://www.gnu.org/software/grub/manual/grub/html_node/sear...

And make sure you've insmod'ed any modules you need for your hardware if necessary (storage, USB, etc.)... unfortunately documentation on these is poor, and some modules are mutually exclusive, so have fun.