In the last 2.5 years Team Kodi has been slowly and successfully driving the Linux codebase towards common standards (GBM/V4L2) and away from vendor-proprietary interfaces (VDPAU, Amcodec, iMX6, OMXplayer, etc.) so there is low interest in adding another nvidia vendor proprietary interface (NVDEC) to Kodi. It's probably not hard to do, but someone has to do it, and even if it's done it's unlikely to be accepted into the codebase.
My assumption was that since it is supported by ffmpeg, provided the correct driver is in place (Nvidia binary blob?) it would just be a matter of changing the command line options passed to ffmpeg.
I'm probably horribly oversimplifying things though.
I know my way around *nix systems, but I am not a programmer or a developer.