I’ve continued tweaking and working on this and have made some pretty good progress: https://github.com/n8henrie/nixos-btrfs-pi
I’m still trying to figure out booting from a compressed @boot subvolume. I’ve made the necessary u-boot tweaks to boot directly from the subvolume, which I’m psyched about, but compressing either @boot or @nix seems to break things. Boots the first time, but after a nixos-install --root / goes into a boot loop.