Very complete an interesting answer. Yes, flashing the high beam to road users coming the opposite direction is also customarily used here in Spain to indicate police control or road problem (accident, sudden traffic jam, etc.) ahead of those coming your opposite way.Another use of flashing the headlamp can be to warn oncoming vehicles that the police are ahead looking for speeders, though that seems to be rare now.
I've never seem bikers greeting each other with flashing of the high beam. In fact, unjustified use of high beam flashing is considered very rude, as if the flasher was saying "hey, watch what you are doing you jerk!" to the flashed one.