Resharing from @aziz.
Solution can be found in the nextjs-shopify starter, specifically the helper function getLayoutProps: Search · getLayoutProps · GitHub
The missing link that I was unaware of is that _app.js supports something called pageProps, which can be used to pass in prefetched data from a page’s getStaticProps into the app-wide template. So then you can pass in the fetched Builder content to a layout component, like this: nextjs-shopify/_app.tsx at master · BuilderIO/nextjs-shopify · GitHub
Info on pageProps: Advanced Features: Custom `App` | Next.js.
Thanks @aziz!