I have an NPC that I don't want to say hello to anyone she sees, you know those "Greetings, how are you today", floating comments. How do I do that? I'm thinking of making a high-priority quest, making her a target for it and putting a silent Hello line in the conversation tab. Would this work?

Check the Skip Fallout Behavior flag in her AI packages. She won't stop to chat when that flag is set.