Hi.
I've biked the Portuguese camino almost twice.
The first time it never occurred to me to take a bell, the second time, because it was short notice (abandoned walking it for the bike), I again didn't not take a bell.
I was more aware of the issues walkers have with us bikers and took care slow down, to whistle or say "bom caminho" before hand. Almost always this still startled walkers. They were appreciative for the most part.
I will be biking it again this August and this time we're taking a bell and maybe even portable speakers for my ipod, so hopefully we'll be the walker friendliest bikers out there
I think its also important to realized that for the amount of traffic and testimony of bad experiences, there seem to have been few accidents. On the camino, at least in the Portuguese it seems to be mostly mountain bikers. MTB has been very popular together with local MTB races. So bikers are used to taking small gaps and keeping a line, travelling at speed. In the races its good etiquette for slower bikers to take the rougher ground if a fast biker is coming behind, this way the overtaking is done more quickly.
I'm not trying to excuse bikers, and totally understand that it very easy to be startled by bikes. It is up to bikers to understand they must change behaviour because the context of their travel is different from a race. It is up to everyone to be courteous and accommodating (within reason) to their fellow pilgrim.