I have wondered about this myself. Watching F1 blue flags are going all the time. Having worked all three tracks this year(my first year) I think some tracks would be easier to do this on than others. HPR corners can't see much if any of the track beyond their section. I would think it would require either tracking equipment at the corners or maybe a separate radio channel with one person at the corner pretty much dedicated to that, working with the rest so they knew whom to blue flag. I like the idea but I think it would be difficult to implement reliably.
We need a live timing app like the F1 app.