But I could easily see nixpkgs implement functions that allow nixos-rebuild switch to use either live patching method, or even implementing one specifically for NixOS.
Sadly, I could not. Live patching requires extensive knowledge of the previous system state and that is the antithesis to NixOS where any system state is fully independent of any other possible system state.
nixos-rebuild switch isn’t very magical at all once you understand this principle.
Live patching is also not really something you want to use or use frequently. It’s more intended for “this super critical box can only be taken down next Saturday but there’s a fix for a 0-day in the kernel today that we need ASAP”. If it’s at all possible to simply reboot, simply reboot (or kexec).