I’m currently looking to achieve a gradient effect a bit like the rectangle in http://pjnicholson.com/Fireworks/fillgradients.htm

If I compromise a little I can get close to this using RadialGradientBrush… but is there any (not too painful) way to achieve the rectangular effect?
Use an
ImageBrushinstead and use this image (or a similar image generated using some image editor) for the background of your rectangle.