I'd like to install NixOS to a flash drive, and have the operating system run entirely on the flash drive. Is it possible to install to a flash drive using NixOps? Or would I even need to? That is, would it be easier or better just to write a script that would do it?
First let's get this out of the way: if the purpose is just creating a live installer to install NixOS on a system, you should follow the instructions in the NixOS manual.
Assuming you want the drive to be capable of updating itself, you should avoid NixOps. Unlike the nixos-install command, NixOps was not designed to copy only the evaluated and built machine to the target machine. It will not copy the Nix-language expressions over to it, so the target machine will not be able to update or modify itself.
I see two possible approaches for this. The most straightforward one is to use the nixos-install command to install NixOS on the drive like you would install it on an internal drive. To the kernel and every sane BIOS, a USB stick is just another block device to boot from.
The other approach is following the manual and changing the root= parameter, which may make changes persistent. I have never done it for my installation USB sticks, so I don't know whether that really works.