That sounds incredibly useful, and it doesn't sound like it should be particularly hard to implement in modern systems. Maybe it just needs a term so people can search and advertise it.
Being able to buffer a video signal so that data can be saved from before a triggering event happened is not a new idea. One of Sony's cameras FS700 I think, could only record a few seconds at 240fps, and then stop. But it had a end trigger mode where it would just keep a buffer so you could press the button after the event (think after you see the lightning strike), and then it would just dump the contents of the buffer up to the point you hit stop. Same thing for sports, hit the button at the catch. Much easier than anticipating starting in time.
Essentially, the same concept, just need enough of a buffer to allow for the pre-roll which wouldn't be a lot at the lower bitrate IP data coming from the cameras
