I definitely know Fox US fires trigger pings to affiliates for things such as local watermarking.
Yes - Fox is the odd-one-out in the US markets as it provides affiliates with a pre-encoded 'network' feed rather than a high bitrate, high quality mezzanine 'contribution quality' feed. The Fox stations all have 'splicers' installed.
These splicers take the pre-encoded MPEG2 network feed, and do a very clever partial decode to insert a local bug, by decoding just the macroblocks in the stream that need to be decoded and re-encoded to do so. The rest of the picture just passes straight through and is not decoded and recoded (avoiding further compression artefacts) This also allows Fox to tightly handle 5.1 Dolby audio on their network shows I believe - which some other networks historically struggled with (or more accurately their local stations did)
The Fox splicer also has an MPEG2 encoder for local material played out from the station (commercials, local news etc.) which is synchronised to the splicer's incoming network feed, allowing clean junctions to and from network. This gives Fox tighter control, mandates a uniform bug insertion system that can be remotely triggered etc.
This system was introduced when Fox switched from running a 480i 16:9 SD network feed - which was encoded at the local stations in 480p - and introduced 720p HD. (Fox - unlike ABC, NBC and CBS didn't initially run HD on digital OTA. Their 480p was 'good enough' for them)
This was the situation a few years ago - I think Fox may have since upgraded their splicer tech further.
ABC, CBS and NBC work differently and distribute a higher quality network feed BUT this is then permanently decoded to baseband video and then re-encoded after playout master control in each station. This requires more equipement and personnel than Fox stations I believe.
Bingo, I've had it explained to me before and it's gone in one ear and out the other, however your explanation is roughly what I've heard previously. It's seriously impressive tech.
What I always wonder is how local stations seamlessly merge their local news 'previews' into a national network ECP
, along the lines of the preview BBC One viewers are treated to at 5:56pm before the Six. I know local NBC affiliates have been doing that since the late 90s, and it always looks so slick.