Custom Visuals are a great idea, and awesome differentiator when compared to competing products. They bring a ton of new and experiential use cases. To increase adoption and awareness, please promote certified custom visuals in Power BI by featuring these visuals more prominently within the Power BI ecosystem (desktop, service, office store).
To do so, you might:
Call certified visuals out in the office store
Add a dedicated tab in the office store and Power BI custom visuals UI
Make "certified" visuals searchable (use terms such as "certified" in search box for visuals)
Add a "certified" badge to promote visuals that have achieved formal certification