Global Product Categories for MarketPress Posts

The best that I can see it, right now each individual MarketPress store owner can choose Product Categories for the products in their own store. I'd like to keep that and also have store owners choose a Global Product Category for each product they post, so I can show a listing of items in the network per global category. (Please note that this is different than showing a global list of all the individual store categories themselves.)

I figure there must be some combination of currently-existing WPMU modules that would accomplish this fairly easily. Any ideas/suggestions on how I can go about doing this? My thanks in advance --