User defined order for product listing in MarketPress

At present the options are for sorting by fields that have other uses, such as title, price or published date. Then there’s the weird one of ‘ProductID’ which is sort of by date and sort of random. It would be most useful to have either a ‘sort order’ field on products, or what would work for us (and would probably be useful anyway) is to sort by SKU (Stock Keeping Unit). Many will be using some kind of structure these these that will lend itself to sorting. For us we use YYNNN format.