I have a feeling  this post may have just answered my question.

I don't know if in the current alpha there is a way to get access to the texture once you have set it in a retained PShape. I tried it, but changing the original texture doesn't affect the PShape's texture after it has been set. Without access to the texture, the alternative is to not use a retained PShape, but do it directly (aka the old-fashioned way).

Code Example

1. PImage original, tex;
2. int w, h;
3.  
4. void setup() {
5.   size(640, 360, P3D);
6.   original = loadImage("yourImage.jpg");
7.   noStroke();
8.   w = width;
9.   h = height;
10. }
11.  
12. void draw() {
13.   float amt = (frameCount / 200.0) % 1.0;
14.   tex = getDesaturated(original, amt);
15.   beginShape(QUADS);
16.   texture(tex);
17.   vertex(0, 0, 0, 0, 0);
18.   vertex(w, 0, 0, w, 0);
19.   vertex(w, h, 0, w, h);
20.   vertex(0, h, 0, 0, h);
21.   endShape();
22. }
23.  
24. PImage getDesaturated(PImage in, float amt) {
25.   PImage out = in.get();
26.   colorMode(HSB);
27.   for (int i=0; i<out.pixels.length; i++) {
28.     color c = out.pixels[i];
29.     float h = hue(c);
30.     float s = saturation(c)*amt;
31.     float b = brightness(c);
32.     out.pixels[i] = color(h,s,b);
33.   }
34.   colorMode(RGB);
35.   return out;
36. }

I had a feeling this was the case, thanks very much for your time, i'll try it, with any luck the performance won't drop through the floor, much appreciated!

Well, there are some optimizations possible. Such as, only call getDesaturated() when you change the amt or use it only for the in-between amt (and the retained PShape for the regular state).

Personally I prefer to use GLGraphics until a stable and documented Processing 2.0 is out.

*edit* Ah, I see you have an alternative option from andres for the upcoming alpha, so good news.