In this case, since the fill is white, you can use a black and white PGraphics + blend().

For different fills or images etc. you could also use mask().

Code Example

1. PImage img = loadImage("http://www.portraitqueen.com/media/oil-abstract-painting-transformation2.jpg");
2. PGraphics pg;
3.  
4. void setup() {
5.   size(img.width, img.height);
6.   pg = createGraphics(500, 170, JAVA2D);
7.   pg.beginDraw();
8.   pg.smooth();
9.   pg.textFont(createFont("Arial", 150));
10.   pg.textAlign(CENTER, CENTER);
11.   pg.background(255);
12.   pg.fill(0);
13.   pg.text("HELLO", 0, -20, pg.width, pg.height);
14.   pg.endDraw();
15. }
16.  
17. void draw() {
18.   image(img, 0, 0);
19.   blend(pg, 0, 0, pg.width, pg.height, 50, 120, pg.width, pg.height, ADD);
20. }