[mythtv-users] I've asked on ffmpeg and mplayer irc channels....perhaps someone here will know...

[Tyler T]

Something roughly similar to:

dd if=my_h264_stream.mkv bs=1M skip=100 count=1 | ffmpeg -i

will skip a ways in and then send a chunk to ffmpeg for analysis.
Tweak skip= to suit; count may also need to be a bit larger.