So, if you are an effective altruist who includes cows and chickens in the potential list of the entities who should be protected from suffering what do you do? Write blogs aimed at an extremely limited audience who do not appear to be overly receptive, anyway? That's not very "effective", is it?

I don't think this gives due respect to the premise. Imagine yourself in a world where attitudes towards meat eating were similar to ours, but the principal form of livestock were human. You'd like to reduce the number of people being raised as meat. Would arguing your ethical position on a site called LessWrong be worth your time, even if most people there weren't very receptive?