In all seriousness, I'm torn about calling "on your left". If I'm on a MUP, I simply accept the fact that I won't be going as fast as I would be on the road, and I slow down until I can pass pedestrians with a wide margin.

A few years ago, I was passing other road riders on a group ride. It was a club-sponsored metric century with about 3,000 riders on various courses. Someone who I had passed on a downhill announced somewhat indignantly, "There's a rider passing without calling it out." I didn't want to say "on your left" because of the large number of riders on the road (would they know who I was speaking to?), as well as the fear that they would veer left in to my path. I was not buzzing people, and as someone (umd?) noted above, if they can't handle being passed with a safe margin, should they really be riding on the road?