Look at it this way.
There are millions and millions of stock photos out there right? It's a $5Billion industry. Most of those photos, not all but most, were set up properly at one point by a photographer, on his own dime, and did a full shoot hoping that one day someone, somewhere will need this photo. So people are already doing this even with extremely bad returns.
People yes. Pros, not so much. This is perhaps one of the biggest problems with your business model. Your target clients are Art Directors and people who expect a professional, high end product, right?
If before their photo shoot, they let other know about it. They will have an opportunity to shoot the photos they already know that someone needs and will pay for it.
So we are back to a pro shooting stuff for you when they are being paid by another client, and all the problems that entails.
So things to consider:
1. A pro most likely won't touch this with a 300 foot pole, those that do are likely killing his own business as a result. You just don't sell photos to someone else when your client is footing the bill for the shoot. Bad business all the way around.
2. You'll probably get plenty of interest from amateur's and fauxtaugs, but they won't produce quality results, at least not reliably. That means vetting every submission, or losing serious face with your clients when they request a picture of a bowl of fruit with some uncooked meat and what they get is something their own kid could have shot with an Iphone for them.
3. So far in this conversation we've gone from a pro doing this on a paid shoot to no a pro shouldn't be doing this on someone else's paid shoot back to a pro doing this on someone else's paid shoot as being part of your business model. That's going to make this whole thing pretty tough to market, don't you think? In all honesty I think it would really help for you to sit down and think about what your actual business model is going to be, and the sort of photographers you'll be able to attract as a result.