I'm afraid i never worked for a product targeted at start-ups before, so i wouldn't know how to make my work sound valuable in such cases. The product i've marketed targeted large companies, so i'd usually use their names to show off the product's marketing work.
In a start-up case, i'd try to prove that the product brought the value it promised (therefore the marketing was accurate), and that people felt compelled to know more when they heard about the product for the first time, whether from you or not (therefore the marketing was effective). If you can get testimonials from your past clients where they testify about that, then you're probably making your case stronger, even if the start-ups aren't known. If you can find metrics of how many people reached you to ask information about the product, then it's probably good too.
I know, easier said than done, but i can't think of anything else on the top of my head.