Looks like I might be out of luck then? My monitor doesn't seem to like interlaced modes very much, here's the default 640x480@60i modeline from arcade_osd and my modified one with a V Total of 501 that works:
modeline "640x480_60 15.68KHz 59.98Hz" 13.05 640 664 728 832 480 483 489 523 interlace -hsync -vsync
crt_range 15673.78-15693.78, 50.00-60.00, 1.839, 4.905, 7.970, 0.191, 0.383, 2.168, 0, 0, 240, 135, 480, 271
^ Default
modeline "640x480_60 15.02KHz 59.98Hz" 12.50 640 664 728 832 480 483 489 501 interlace -hsync -vsync
crt_range 15014.04-15034.04, 50.00-60.00, 1.920, 5.120, 8.320, 0.200, 0.399, 0.799, 0, 0, 240, 140, 480, 280
^Works
If there's no way to lower it I guess I'm out of luck, except for specifying that mode directly in the game.ini files? Some games use interlaced where I wouldn't expect it, like asteroids and space invaders. Is there a way to prevent them from using interlaced modes altogether? I doubt many games I plan to play were designed with interlaced modes in mind