Sorry, Matt has been rather busy the last week and has not had time to respond to your question, so let me try.
Snow on a sink is almost always caused by the sink displaying encrypted video. Many sink are designed to go blank if the stream is encrypted and it can not decrypt the image. This sink apparently doesn't do this so it shows snow.
Depending on the source software (in this case the ADV7511), it might transmit encrypted video even if the HDCP is not authenticated, which is needed to display snow on the sink. You can program the source to not transmit until HDCP is authenticated. Most DVD players will not transmit video until HDCP is authenticated.
0xD5[0] = 1 should output black images. You can set this until HDCP is authenticated.
Another way to do this is with AVMUTE however this is dependent on the sink handling AVMUTE correctly.