I just had a play with this to see if I could do it in FCP. Here's my recipe.
1. Move the green screen footage up to V2.
2. Make a copy of the green screen footage with option-shift and slip it up to V3.
3. Put your background on V1
4. Key out the green on both clips.
5. Using the distort tool, manipulate the image on V2 to lay where you would like the shadow to fall.
6. Now on V2 apply the following filters -
Pete Warden's Posterize (free at www.petewarden.com) Level set to 1.
Desaturate 100%
Gaussian Blur - I used a radius of about 14.
Probably a lot more fiddly than a plug-in, but free and simple enough.
Of course if the talent passes in front of an image the shadow won't distort around it, but for a fairly static image it works fine.