There is a Syphon screen capture app but it's still in beta. If you know how to compile an app in Xcode you can download the source here:
http://code.google.com/p/syphon-framewo ... e/checkout. Unfortunately the app can't isolate a window so any overlapping window will be shown.
The dirty way would be to use a screen capture app like CamTwist (
http://camtwiststudio.com/) to turn the Keynote window in to a camera input. That is most likely slower and with less quality. But it's worth trying both methods since they are free.