I did user testing by prototyping the video chat with the photo boot application on my mac that people were recorded to, and combined it with simple, small yellow notes with smiley faces on them. People assigned to simulate a conversation on video chat and try to use those smiley cards if they fell that they want or need to use them.
I got very interesting results and responses, one of the most interesting details is that no one covered his face with the card, they showed it to the camera, at the bottom of the screen, from the side but never covered the face.
People were amused by the idea but very strongly preferred their own face as the emotional expression, but at the same time they liked the idea to be able add some external graphics to the video chat, something that will complement their conversation, but not change it.
To summarize the prototyping: People are interested and open mostly to additional visuals in their video chats but at the same time they prefer their real face for emotional transformation.