Realtime time overlay with ffmpeg
Wow... ffmpeg never ceases to amaze.. it's built in!

ffmpeg -re -loop 1 -i smpte-color-bars-1080p.png -vf drawtext="fontfile=monofonto.ttf: fontsize=96: box=1: boxcolor=black@0.75: boxborderw=5: fontcolor=white: x=(w-text_w)/2: y=((h-text_h)/2)+((h-text_h)/4): text='%{gmtime\:%H\\\\\:%M\\\\\:%S}'" -r 25 -vcodec libx264 -f mpegts udp://127.0.0.1:1234


The important bit for the timecode being: text='%{gmtime\:%H\\\\\:%M\\\\\:%S}'

There is some crazy escaping in there to get the colons to display(in bash), but that is UTC/GMT time of day timecode burnt in... just need to figure out the syntax for multiple drawtext functions now and I'm all set...