with an additional API call.

That's a way.
But I just remembered what I saw in the logs inside the container:
The promoted posts are updated periodically. Perhaps they are already available in the store and you don't need to make an additional query. But I haven't checked it now.

Extract from the log:

Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0
Loading special posts
Loaded special posts: featured: 0, promoted: 0, notices: 0