there's a program that can modify ps2 isos to force 480P. Doesn't work on every game though. Do some googling and you'll find it.
I wish there was a hack to force 480p on already burnt games, as I don't have the HDloader, and I don't fancy re-burning my 50+ games again.
It's annoying cuz I want to play my ps2 hooked up directly to my lcd monitor (im running my wii and 360 through it) but it can't display interlaced signals. I can play the few games I have that have 480p option, but a lot seem to be enabled through in game menus, which is useless when I can't see whats on screen UNTIL it's in 480p.