After doing some more poking it appears as if Avisynth (and thus NNEDI3) is Windows-only. Do you happen to know if there are ways to run it in Linux or OSX? Or if there's a comparable set of software for those platforms?
Avisynth should run in Wine, but there is also Vapoursynth[1] (which works natively on OSX & Linux) and a NNEDI3 port[2] for it. After getting both of them up and running, a script like this[3] ran with the vspipe program that comes with Vapoursynth should do the trick. It's a bit cumbersome since Avisynth and Vapoursynth are primarily intended for processing video, not images, but it gets the job done in absence of a dedicated NNEDI3 resizing tool. I'm actually using this exact setup at work myself when I need to do any image upscaling.
