Don't do it at the encode level - do it at the container level. Modern containers like mkv supports setting the pixel aspect ratio when you mux the video. So encode the video ay 720x480, then use that PAR option when you mux.
I'm not very familiar with mkv muxing, but for mp4 muxing using mp4box, it's like
Code:
MP4Box.exe -add Video1.264:fps=23.976:par=32:27 -add Audio1.m4a:lang=jpn Output.mp4