I also NEED a solution to this. Right now a promo code can be either set for unlimited use or for one use per registrant. And we can limit it to a specific listing if desired.
But we need the option to limit how many times a specific promo code be used, even if limited to a single listing. I need to be able to say "this promo code can only be used once." I literally just had this need and I had to watch for a specific person to enroll and use the code so I could disable the code after they enrolled.
I think it could be helpful if we could also limit a promo code to a specific user/email address.
I know there's also been discussion about tiered pricing or discounts based on some kind of prerequisite. I think a promo code limited to a specific user could go a long way towards that. For example, if you wanted to give people who have previously taken a course a discount, you could assign a promo code to their account and email it to them so they can take any future courses and use the promo code each time. Of course there's flaws in that as well, like say they change their email address for some reason. But those are kinks to be worked out.