I am actually busy with the exact same thing. What I realised is that you can add extra media to an image on the product tab. The debug mode hinted me to the data model <product.image>. I think <product.image> can be used to store multiple images and to then relate it to the <product.template.id> or <product.product.id>. Have not tried it yet myself (will do in the next weeks I guess) but if it works, then there is actually no need to add a new field to store multiple images per product. Just use the <product.image> model to store the image.