This is an area we can consider for the future, but I would say there's probably no need for a drop-down or method of marking. A product is automatically a back order when it's pending with no stock, marking it as such would be an unnecessary step. The reason we haven't put an automatic email in before is that it's really a personal choice per retailer as to how they want to handle orders which can't immediately be satisfied.
If the system is working the way it should, the customer should be aware at the point of order that a particular item is either low in stock or out of stock - and that they're effectively submitting a back order. I'd appreciate a little more information on this one regarding the kind of email your sending out, and when it's being sent out. Some feature improvements, such as this one, have to be carefully considered so that they can be made to fit the general requirements of the widest range of users.
Developer :: Bluepark Solutions