I think that there should be a mechanism to write a review on the winning entry and other entries only after the contest is closed and the designer got paid.
Everyone knows that the winning design is not necessary one of the best designs in the crowd, the outcome is a sole discretion of the CH, it depends on a lot of factors, but in some cases it might only depend on CH's understanding of simple graphic design principles. I think it is beneficial for both the designers, returning CHs and new CHs. They might browse those reviews and get something out of them.