I did something similar as 10 Wheels did with an Incredibell, but on the right side (decided I wanted more dexterity for the front brake). Allows me to bell AND brake (more or less) w/o moving hands too much. I've found that a significant amount of time, "on your left" generates a left turn and deer-stare from walkers. A bike bell is unambiguous regardless of language or age and seems to get the walkers and joggers with earbuds in out of their zone and back into the real world. Seems to help a little warning off deer and other critters.

I also have left the front brake pads on my commuter (cantis) deliberately squeally. Bike brakes squealing HARD behind someone is 100% unambiguous. (I don't want to hit them, hurt them, or even scare them. I just want them to decide which half of the bike trail they are going to use for the moment and stick with that decision until I've passed them.)