AIUI, Pixel Magic Systems are developing an HD-SDI mod kit for use on BD & HD-DVD players (and maybe other HD sources). That would take the decoded and up-sampled 720p and 1080i YCbCr 4:2:2 digital video signal directly from the output of the MPEG 4 (and 2) decoder, and feed a Crystallio 2 or Vantage etc scaler in the same manner that SD-SDi does today.
Because all the MPEG4 storage is kept on the disc side of the MPEG decoder, it doesn't matter whether the BD or DVD is encoded in MPEG 2 or 4 - SDI has no knowledge of that, it is just a digital component interface. HDCP is also irrelevant, because the encryption is applied later by the player.
HOWEVER, as mentioned Blu-ray etc players may not have the same access to decoder output pins as DVD players, and the decoder may do other video processing (like the current Sky+ box) so there may be no unprocessed video output. In addition, that output would be at a much higher data rate than even frm DVDs - about five times higher. So all the installation problems associated with high frequencies that people have been having with SD-SDI installations will be greatly magnified.
I live in hope, but HD-SDI may be a lot to hope for.
Nick