I'm a big believer that you get what you give. Respect and consideration for others should be a way of life and it shouldn't be dependent on reciprocity. Safety isn't dependent on the number and variety of people using a MUP, it depends on everybody treating each other like we have each other's backs instead of like we are in each other's way.
As far as what to do or say when passing, on a public path I like "Excuse me, passing on your left" as "Excuse me" is both polite and universally accepted as an indication that someone wants to get by and "passing on your left" lets them know what's coming. When on group rides or other organized cycling events, I just opt for the traditional "on your left". You have to give your warning early enough in advance to be of benefit, even if that means slowing down a bit on a public path. I find it silly when someone yells "ON YOUR LEFT" in your ear when by the time they they get the words out they are already past you. They might as well yell "I JUST PASSED YOU ON YOUR LEFT". It's like a driver turning on his blinker when he is already half way through the turn.