Show the quantity field in product list not just on single product page

I would like some pointers on how to get the quantity field in the product listing page if that is possible. The reason for this is my client is a wholesaler. It would be good if his regular customers can just go down the list and type the quantity they want of each product and click add to cart with no page reloading until they change category or whatever. Rather than having to go to the single product listing to type the quantity or add 1 of each required product to the cart then view the cart and update the quantity for each item in there.

I did get an initial response on this question from Phil here but I did not pursue it further at the time. WP-Ecommerce has this option out of the box so I tried that but I'm a bit sick of hacking WPEC to try to get it to work in other areas so here I am back again.
So here are my questions:

1. Can I do this with custom MarketPress template files?
I assume from marketpress-includes/themes/Themeing_MarketPress.txt that it would involve creating a custom Product List Page (mp_productlist.php) or similar.

2. Do I have to create this file from scratch or can I copy the standard one from somewhere?

3. If I create it from scratch which of the functions from /marketpress/marketpress-includes/template-functions.php would I have to include to simulate the standard /store/products?

4. Am I on the right track here or have I missed something fundamental?

5. If I can do this would there be any dramatic speed or server load issues with generating a quantity field for 20 items per page?

Thanks for your help.