Does anyone know if it is possible to set up a promo code in Store that only applies to products in a certain category?
I am happy to use an extension (if one exists) to fill the gap if this isn't supported natively.
There are no existing extensions that I'm aware of that can handle this functionality. Currently promo code discounts are applied to a whole order, not individual items.
However, it would certainly be possible to write something in PHP to handle this using the extension hooks.