What do you do if you make a B2B product that gives your customers such a big competitive advantage in the market that they won't talk about your product, actively hide that they use it, and refuse to let you tell others that they use it?
Have you considered changing your pricing model so you get paid some % of their "success" however defined? Edit: to explain my thinking - the idea here is it may actually be that if you're getting paid alongside them that you don't need to grow to other customers. You can get all the revenue you need from getting this one customer to dominate their market.
Edit. I should add that I have offered a discount in return for being able to publicise their use. Unfortunately, everyone has declined the offer.