1. Boot from the Debian
ppc64 Netinstall ISO linked above
2. Choose "Default Install"
3. Run through the simple prompts in the installer, use the full disk during partitioning, and choosing all defaults is fine.
4. When you get the message about GRUB install failing, IMMEDIATELY press Alt-F2 to open a console, and hit enter to start it.
5. Type the following:
chroot /target
wget
https://repo.powerprogress.org/debian/install/grubfix32.sh
chmod u+x grubfix32.sh
./grubfix32.sh /dev/sda2 (or whatever your partition was called during partitioning)
6. If you see "GRUB installed with no errors reported" in the wall of text, you're good! Press Alt-F1 to go back to the install, and choose "Install without bootloader"
7. Run through the rest of the prompts, and reboot into your shiny new Debian install!