It is not possible to override the fetching of tiered prices in the plattform today. It would be good to have to option to override but for example decorating a service.
A use case for this is when product prices is fetched from an external system, such as an ERP (as described here). If the price calculator is decorated prices will be calculated according to the new logic. It is however not possible to change the way tiered prices are fetched and presented in the storefront API.