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I think I had a similar request and the recommendation from AW was:
You can use the Customer – Update Custom Field action with the customer.generate_coupon in the Value field. Here’s a screenshot (http://pic.pros.pr/46ec63c1b23b) of what the rule would look like. Then this workflow runs, it would generate and save the coupon as user meta. Then, to use that coupon (in an email to your customer, as an example), you’d use the customer.meta and pass it the key you used in the previous action (http://pic.pros.pr/fadf6ad9904a)