Currently, replacing NIX_INSTALLED_NIX in the install script is required, so I wrote the following script, based on the one written by @ChadSki above:
#!/bin/sh
# Based on https://discourse.nixos.org/t/getting-started-with-nix-on-ppc64le/12712/8?u=jiegec
nix-build '<nixpkgs>' -A pkgsCross.powernv.nix
# Get a donor copy of the installer
wget -c https://releases.nixos.org/nix/nix-2.11.1/nix-2.11.1-x86_64-linux.tar.xz
tar xf nix-2.11.1-x86_64-linux.tar.xz
# Keep the cert package
mkdir -p nix-ppc64le-linux/store
cp -r nix-2.11.1-x86_64-linux/store/*nss-cacert* nix-ppc64le-linux/store/
# Toss other packages, keep the scripts
rm -rf nix-2.11.1-x86_64-linux/store
mv nix-2.11.1-x86_64-linux/* nix-ppc64le-linux/
# Add ppc64le packages
cp -r $(nix-store -qR result) nix-ppc64le-linux/store/
# Generate a .reginfo for those ppc64le packages
nix-store --dump-db $(nix-store -qR result) > nix-ppc64le-linux/.reginfo
# Replace NIX_INSTALLED_NIX in install script
export NIX_INSTALLED_NIX=$(readlink result)
sed -i "s#NIX_INSTALLED_NIX=\".*\"#NIX_INSTALLED_NIX=\"$NIX_INSTALLED_NIX\"#" nix-ppc64le-linux/install-multi-user
echo "Done! Copy nix-ppc64le-linux to ppc64le machine and run ./install --daemon"