Potential Solution
I’ve discovered that Avid now supplies an Avid DnxHR format that works with 4K. When I tried custom setting the resolution to 2560×1440 with DnxHD it only outputted 1080, but I tried it with DnxHR and it worked, so it seems as long as I don’t surpass 4K I can custom set the resolution in DnxHR. The only issue is the file size is still rather large. It is however almost exactly half the size of the ProRes files I got out of camera. Unless someone comes out with a better solution I’ll go with this.