Multi-category discount

Hi,

I was wondering if anyone had any ideas of how it would be best to implement a multi-category discount? In my specific case, a customer makes a purchase from each of 3 categories and then gets a 10% discount on these purchases.

Using a Coupon, I would have to allow the 10% discount to the entire cart OR select all the products, from those categories, and there are going to be hundreds of products - so it'll be easy to make a mistake, when selecting products in the multi-selectable combo (a grid with a leading check box would be a much nicer way to do this). Plus when you add a new product to one of these categories you have to remember to update the Coupon.
If the 10% of everything was a valid option, I would still need to validate product categories. To do this I think I'd need to change cart.php in library/cart to retrieve the categories and pass them back to catalog/controlller/cart.php so that I could validate the categories? I assume library/cart/cart.php is a core file, so should I really be changing it?

If I modified the Coupon to list "Valid categories" (like products), I'd possibly need to add a switch to indicate whether the coupon was used for products or categories? There are still lots of other changes I'd probably have to make (like everywhere there's a product check).

Not sure I can use the discount processing.

Any ideas for modifications (and/or workarounds) would be much appreciated.

Regards,
Phil