To make a long story short, I'm trying to capture video for a Let's Play series, and I've worked out every little thing, except that I need to adjust the PS2's volume at a system level, rather than through the TV because my capture set up is kinda weird. Does anyone know if there's a way to do this, or is it impossible? If it is, is there some kind of cable with in-line volume control that I can add to the PS2's audio cables? Or am I just totally boned here?

I think every cheap headset have a potentiometer like that and it's for two channels. The only thing that may be problematic is the soldering, but you can get away with just twisting the wires. So this would be the easiest way to do this.