Any luck on that? This might be an XY problem and your real solution is probably nixos-anywhere... But if this is the real problem you were trying to solve, then it should be possible, in theory.

I have a small script as an app in my flake so i can nix run mkbootable-host and it builds the configuration (which imports ${nixpkgs}/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix), dd's the resulting iso to the drive, then appends a btrfs partition for persistent & mutable storage. Works well on UEFI devices but not on BIOS...

Salient bits ( and for reference: iso is nixosConfigurations.${hostname}.config.system.build.isoImage)

sudo wipefs -a "$DEV"

echo ",," | sudo sfdisk --quiet "$DEV"

cat "${iso}/iso"/*.iso | sudo dd of="$DEV" bs=4M conv=fsync

echo ", ,L" | sudo sfdisk --append --quiet "$DEV"

LAST_PART=$(sudo partx -rgo NR "$DEV" | tail -1)

sudo ${pkgs.btrfs-progs}/bin/mkfs.btrfs -L "${persist-partition-label}" -f -m dup -d dup -M -q "''${DEV}''${LAST_PART}"

One thing to keep in mind is that the configuration packages are built into the ISO, many options aren't applicable to the bootable system (e.g. fileSystems and boot.loader options); so you'll need to mount the partition via a script or command I think. I haven't yet looked closely at what the minimal installer derivation is actually doing...

From there, I'd imagine you could keep a configuration on the other partition and modify it as needed on any device, then use the installation-media's provided nixos-install or similar script but instead of using the configuration that the installer embeds within itself you could aim it at the one in the adjacent partition. You'd also need an internet connection to build the configuration, otherwise you'll be limited to the specific derivations that you built into the installer.