The only way I could imagine getting around this is to trick the xbox into thinking your TV is bigger than it is. FFXI resolutions is changed based on the xbox dashboard's settings, so if you play on a SD TV, it'll end up looking like the PS2 version just smoother. When I had to use myxbox to play, I had the xbox set on widescreen (on my HDTV) and the game adjusted the interface to compensate...
EDIT: Now that I think about it, it'd really distort the aspect of the screen on a 4:3 TV lol, which would be worse than what you have.
Sorry if that didn't help, but reducing the max lines shown really is the only way to get more screen area other than getting a different TV.