shadow only (with yafray)

Hi all,

I decided to give yafray a try for something I’m working on but I have a problem.
With the Blender Internal Rendered, I can add a plane and switch ‘only shadow’ so that I can get a solid coloured background, but with yafray, this options isn’t present.

How do I go about doing this with yafray?

no direct support, but there’s a hack:
(personally have never tried)