To be honest, I think this "feature" is a distraction and I'd argue strenuously against it.
We can look at this request in terms of prioritizing the call to action and communicating clearly based on the desired result.
- What is the primary goal of a login page? (to get logged in)
- Does cross-marketing products on that page benefit the user in any tangible way? (no)
So why would we do it?
Let's step through how the feature might work, if the client is dead set on advertising non-login related calls to action on the login page. Never mind that they've confused their UX priorities. If your client insists on doing this, I'd propose to solve it as follows.
- List the products in a grid below the login
- If a product is clicked, outline it, and add an alert box saying "Thanks for your interest! You will be taken to this product after successfully logging in.
- Allow the user to complete their login, and forward appropriately.
This behavior is still problematic though, because a login page typically redirects to the user's last location. Changing that behavior risks confusion.