If the collab product is apparel, it just makes sense to model it. If the product is a swimsuit or undergarments, and you don't feel comfortable modeling it, reconsider applying for the collab. 

There are exceptions where brands do not expect you to model the products, and if that's the case we'd let you know under View Details. We only want you to do what you're comfortable doing! Therefore, you could come up with a conservative way to show them off or you could just skip that collab and not apply.