That sounds like the best way to do it that I can think of.

You can save some effory by using textWidth to make sure your PGraphics(2,3,GL) aren't way too big/small so you only generate the minimum data required to do it.

Just knocked this up quickly, seems to work like you'd probably expect:

Code:

PGraphics3 grad;
PGraphics3 txt;
PFont f;
PImage res;
void setup()
{
  size(400,200);
  f=createFont("Arial",32);
  textFont(f,32);
  float w=textWidth("MyText");
  PGraphics3 grad=new PGraphics3((int)w,50,null);
  PGraphics3 txt=new PGraphics3((int)w,50,null);
  grad.defaults();
  txt.defaults();
  grad.background(random(255));
  for(int i=0;i<200;i++)
  {
    grad.stroke(random(255),random(255),random(255));
    grad.line(random(grad.width),random(grad.height),random(grad.width),random(grad.height));
  }
  txt.background(0);
  txt.fill(255);
  txt.textFont(f,32); // this may not be needed, I don't know
  txt.text("MyText",0,32);
  res=grad;
  res.mask(txt);
}

void draw()
{
  background(255);
  image(res,width/2,height/2);
} 

It's probably wrong in some respects, but gives the effecty you're looking for.

(You'll have to change the fill/random stroke part to do hatever you want to get the gradient.