Discount by location

[djwhyte]

Is there a way to limit a discount code to a specific shipping location? So if you use the relevant discount code you can only ship to one location.  

I'm trying to think of the best way to do this-- it needs to be fairly granular. We want to run a discount for customers who collect from one of our retail locations. 

I even thought about writing a script to alert the customer if they select an invalid location by checking the shipping location against the discount code, but neither the discount code nor the shipping location are surfaced pre-checkout (i.e. in the basket) so I can't do that.