Yeah, for horizontal scrolling (using H mirroring or one-screen) I think the attribute glitch will have to be partially visible.

But for vertical scrolling, almost all NTSC TVs cut off enough of the screen to where it would probably be hardly noticable, if it all, if it was done right. I believe the numbers given are like 240 pixels on screen, but only about 224 are supposedly visible.

That would require some trickery though, because one attribute byte will affect 32x32 pixels. So you'd have to update the nametable 4 bits at a time.