Kate,
  When you go to the zoo to talk with them about their
breastfeeding policies, they should look into what other
zoos do. If I remember correctly, when we were at the
Atlanta zoo a couple of years ago there were breastfeeding
stations for women. I don't remember if there
were any signs specifically welcoming breastfeeding anywhere.
 I think that there are also similar breastfeeding stations
adjacent to but not in the bathrooms at the Roger Williams zoo
in Providence, RHode Island. Maybe some local folks from those
communities can confirm whether my memory is correct.
  I  don't think public discussions on the topic is the way to go.
Just do it! It shouldn't have to be a big deal.
 Don't forget to bring in (or send with your letter) some
sample "breastfeeding welcome here" stickers and/or posters.
  Naomi Bar-Yam PhD