"Passing on your left, please" is pretty cool when it can be heard. Unfortunately by the time it is clearly heard and understood you are probably so close that if an incident were to occur it would at that time.
I still prefer a warning of some sort from a reasonable distance, I do the same when running after scaring the beejeesus out of a couple of people that were walking along and didn't realize I was there.
I've done very very few charity rides though I've wrenched, etc at a few over the years and it is my experience from observations that you get the most people without any group riding or riding experience that are the most apt to be "unpredictable" and make moves that seem bizarre.

When you are on your bike you are in control of the situation and must be the person that acts in a safe and courteous manner for everyone...it is kinda/sorta like "defensive driving" though I prefer the term "offensive driving".