The Antidote to angry Customers is quite simple... Don't be indifferent. Use the power of emotion through:
When a Customer is angry is the precise time to show them that you care!
I suppose from an NLP perspective you could mirror the hostility of your angry customers too. If someone approaches you all upset about something you could be upset too and tell them "what a frustrating day you're having" for xyz reason (nothing to do with them). Then calm down and see if they follow your lead (you know, the pacing and leading approach). Once you have rapport with your customer, talking business should be a little easier, don't you imagine? Do you think that approach would work well? I haven't tried it myself.