When the Planar Projection spawns, start the Hailing.
An organized raid will have the clerics rezzing the unfortunate. The newly rezzed should hail as soon as they have strength enough to get within hailing distance.
Different Planar Projections are on different timers. Some have a 5 minute window, some have a 10 minute window. Also, I have seen the very same mob de-spawn at different intervals. This doesn't happen often, but I most definately seen a mob on a so-called ten minute timer de-spawn in seven minutes. It makes absolutley no difference if you start hailing immediately or if you wait until the raid is rezzed, since the clock has already started when it appeared.
Tell the players who already have the flag, to SIT DOWN facing the Planar Projection, or move out of the way, so the players who need the flag can see the mob.