Just as with chair flying emergencies in advance so we are ready to react, this "monitoring and advising via handheld if we see something bad unfolding" should be thought through... This group of people on PoA is a great place for that to happen.
So, considering that by the time you realize a collision is imminent and your voice on the radio MIGHT help prevent it, what exactly would you say, in what tone, to get the attention of the pilot(s) that need to take action, to increase their separation, and prevent the collision?
Imagine you were in the Arrow here. What EXACTLY would you need to hear over the radio to make you IMMEDIATELY realize that you need to STOP WHAT YOU ARE DOING, arrest the descent, turn left or right, or whatever needs to be done to prevent the collision?
Now, remember that you don't always listen to every call on the radio... So you might not KNOW the type or call signs of the conflicting aircraft when you make your call. What if both pilots pull up or away?