with the technology and protocols available nowadays video jack would be an rtp server(and even rtsp), maybe with rtcp to sync stream as the jackd transport, using sap to connect to it. the problem would be the format used in order to get rid of bandwidth and cpu problems (rgb? yuv? light huffman compression?)
As for sap server there is minisap server from videolan guys.
for rtp there's live.com or the whole project fenice
Why rtp? because it's the Real Time Protocol!
because it's the Session Announcement Protocol and can use multicast.
because it's the Real-Time Streaming Protocol