[ringleader]

Actually, I went off and had a think about what you said Lionel, and I don't think your suggestion is needed for this hack (not in the way I'm thinking about it anyway).

For example: if you create a single coupon, it's valid to be used by anyone who has the code.
So if you create the coupon 'Easter' and email it to certain people for a 50% off offer, they can all use that code. The hack only takes the value of a particular coupon off a persons potential payments; it doesn't discriminate about who owns the code.

Unless you mean to add person-specific coupons to people, in which case they could post theirs all over the internet (the same way as if there was only one code in existence) and everyone who had it could still use it.
It would defeat the purpose of anything if a single code were assigned to a single person, creating a great many queries for your board with little comparative return.

Unless, of course, you mean something entirely different in the first place