I just burned v1.23b and tested it with OSSC. NTSC PAR looked correct with my TV set to 4:3 mode. There was a slight difference between DTV-480p and VGA 640x480p sampling modes in linearity test. It actually looked like VGA 640×480 mode would result to a bit more correct linearity, although it might also be my TV doing something funny. However, 720×480 sampling does not result to 100% correct aspect by definition – I recall you’d need something like 704×480 but that wouldn’t be supported on many TVs. Anyway, I still recommend everyone to use DTV-480p with Dreamcast – checkerboard test shows exactly why.
I tried Video Test debug menu but changing options did not have any effect (except “Draw Video Border”).