It is still region locked. That doesn't change.
But graphics format has nothing to do with region code on PS2. For PS2, NTSC or PAL display is determined based on what game is being played.
For example... A PS2 50hz PAL game not patched to force otherwise, will still use its original PAL video when played on the NTSC PS2 system. Which is annoying because older American TVs won't display PAL.*
In your case, you're running an NTSC game. You want the game to remain NTSC so the TV properly knows how to show it.
*60hz mode of PAL PS2 games uses NTSC color by default. So older PAL TV owners may their fact own problem, a black and white picture.
I'll give you a helpful chart. This is assuming neither patched games or modified hardware. Color is what PS2 systems handle differently than the original PS1 did. As you'll notice.
PS1 NTSC model + NTSC game = NTSC color 60hz
PS1 NTSC model + PAL game = NTSC color 50hz
PS1 PAL model + NTSC game = PAL color 60hz
PS1 PAL model + PAL game = PAL color 50hz
Any PS2, playing an NTSC game = NTSC color 60hz
Any PS2, playing a 50hz PAL game = PAL color 50hz
Any PS2, playing a 60hz PAL game = NTSC color 60hz