Using Minimum Order for multiple products

Hi Everyone
We're getting closer!
I just now realized, d'oh, that the minimum quantity to buy functionality applies only to individual products. We want to set up our store so that customers can purchase 1 or more of individual greeting cards, but must purchase at least 12 cards total. It would be great if we could assign a minimum number of products to buy within categories! Is that possible?

Unfortunately we can't use the minimum order amount in storewide settings since we have pre-packaged card assortments that will cost less than the hand-picked 12-card boxes.

So I'm stumped as to how to handle this.
Any ideas will be greatly appreciated.

Warm regards
ed