Simply make the SD DVD 16:9.
HD implies 16:9 but SD does not imply 4:3. Notice that 720:480 is not equal to 4:3. Making the pixels in the 720:480 a little higher than wide can give 4:3 image proportions. Making them a little wider than high can give 16:9 image proportions. SD 720x480 is flexible about this.
FCP Sequence Settings call this "pixel aspect ratio". For 720x480 video you can choose "NTSC - CCIR601" and check the box for "Anamorphic 16:9".
Dennis Couzin
Berlin Germany