[Inkscape-devel] Extracting a saturation map with SVG filters ?

I can't figure how I could extract a saturation map (as a greyscale image) with SVG filters. My idea is using it to replace any hue of an image with only one like it's done with cyan in the joined png created with Xara Xtreme Linux Edition. I am searching for a long time already but without success. Perhaps not possible.

In Xara I only put a cyan rectangle above the image and apply the "Hue" transparency type. I was able to synthezize all Xara's color effects but this one.

ivan