I found a few resources - haven't tried either. Firstly there is the OpenCV port to AS3 called Marilena here. The site is in Japanese but if you click on the "English" link you'll find the address to their svn repo, and can check out the source. The project comes with a classifier that will detect certain predefined features in images. What I didn't find was the trainer for the classifier. If you can find how to train the classifier this should work.
That seems a bit of overkill though, since all you need to do is detect a green blob in the image, which is a lot simpler than detecting a face. For that purpose this blob detection algorithm should work just fine. You might need to do some tweaking to the code as it is looking for white pixels and you're blob is green. If you're application is being run in a controlled environment (the green is always green) then you shouldn't have too much problems with this. You might also want to make sure that the blob is somewhere in the middle of the image to avoid detecting blobs in the background and so on. Either way a bit of testing with different lighting conditions and different background should give quite nice results.