First, you don't draw the green line from the end of the string, but from the start: one of its ends is on the red circle.

Next, it is unrelated to the string drawing: this one is fully contained in a push/popMatrix, so its rotation is unused in draw, and the value it returns isn't used to rotate this line either (I can move up the line drawing just after the call, without changes).

The rotate(pos.z); line applies the rotation from the last draw letter of the previous string, actually. It is the value used by your green line.
And to draw your letters, you add HALF_PI to the rotation. Perhaps for your line, you need to subtract this value.

1. // Learning Processing
2. // Daniel Shiffman
3. // http://www.learningprocessing.com
4. // Example 17-8: Characters along a curve
5. PFont f;
6. float textHeight;
7. PVector pos;
8. PVector addV;
9. String[] messages = {
10.   "111-W01 | 9+", "222-W32 | 27+", "333-A BIGGER TEXT", "444-Bend to left instand of right"
11. };
12. float[] stops = {
13.   PI, HALF_PI, HALF_PI, -HALF_PI
14. };
15. void setup() {
16.   size(800, 800);
17.   f = createFont("Sans-serif", 40);
18.   textFont(f);
19.   // The text must be centered!
20.   textAlign(CENTER, CENTER);
21.   smooth();
22.   textHeight = textAscent()+textDescent();
23.   pos = new PVector(width/2, height/2, 0);
24.   noLoop();
25. }
26. void draw() {
27.   background(255);
28.   pushMatrix();
29.   translate(pos.x, pos.y);
30.   rotate(pos.z);
31.  
32.   for (int i = 0 ; i < messages.length; i++) {
33.     // Start in the center and draw the circle
34.     if (i == 0) {
35.       stroke(255, 0, 0);
36.       line(0, -height/2, 0, height/2);
37.       line(-width/2, 0, width/2, 0);
38.     }
39.     // show start as red dot
40.     fill(255, 0, 0);
41.     ellipse(0, 0, 15, 15);
42.     fill(0); 
43.     stroke(0);
44.     addV = drawCurved(messages[i], stops[i]);
45.     translate(addV.x, addV.y);
46.     rotate(addV.z);
47.     // show end as a green dot
48.     fill(0, 255, 0);
49.     ellipse(0, 0, 15, 15);
50.     // create some spacing between each 'arc'
51.     strokeWeight(2);
52.     stroke(0, 255, 0);
53.     line(0, 0, 100, 0);
54.     // translate the spacing
55.     translate(100, 0);
56.   }
57.   popMatrix();
58.   noLoop();
59. }
60. PVector drawCurved(String message, float stop) {
61.   PVector returnVector = new PVector();
62.   float startX = screenX(0, 0);
63.   float startY = screenY(0, 0);
64.   pushMatrix();
65.   rotate(HALF_PI); // default direction should be >>
66.   // get the length of the message
67.   float l = textWidth(message);
68.   // the radius
69.   float r = l / (TWO_PI*norm(stop, 0, TWO_PI));
70.   translate(r, 0);
71.   // We must keep track of our position along the curve
72.   float arclength = 0;
73.   // for debug
74.   pushStyle();
75.   noFill();
76.   stroke(0);
77.   strokeWeight(0.5);
78.   ellipse(0, 0, r*2, r*2);
79.   line(0, -r, 0, r);
80.   line(-r, 0, r, 0);
81.   popStyle();
82.   // For every box
83.   for (int i = 0; i < message.length(); i ++ ) {
84.     // The character and its width
85.     char currentChar = message.charAt(i);
86.     // Instead of a constant width, we check the width of each character.
87.     float w = textWidth(currentChar);
88.     // Each box is centered so we move half the width
89.     arclength += w/2;
90.     // Angle in radians is the arclength divided by the radius
91.     // Starting on the left side of the circle by adding PI
92.     float theta = PI + arclength / r;
93.     pushMatrix();
94.     // Polar to Cartesian conversion allows us to find the point along the curve. See Chapter 13 for a review of this concept.
95.     translate(r*cos(theta), r*sin(theta));
96.     // Rotate the box (rotation is offset by 90 degrees)
97.     rotate(theta + HALF_PI);
98.     // Display the character
99.     fill(0);
100.     text(currentChar, 0, 0);
101.     strokeWeight(0.5);
102.     line(0, -r, 0, r);
103.     if (i==message.length()-1) {
104.       // get how much we did move
105.       float endX = screenX(w/2, 0);
106.       float endY = screenY(w/2, 0);
107.       returnVector.x = endX - startX;
108.       returnVector.y = endY - startY;
109.       /*
110.         arclength += w/2;
111.        theta = PI + arclength / r;
112.        */
113.       float rotation = theta - PI;
114.       returnVector.z = rotation;
115.     }
116.     popMatrix();
117.     // Move halfway again
118.     arclength += w/2;
119.   }
120.   popMatrix();
121.   return returnVector;
122. }