ALAsset contains the metadata for an image, which includes resolution, camera settings and location, but it doesn't appear to include comments from a photo stream.
That makes sense, but I would like to retrieve those comments.
Does anyone know if it is possible and where they might be stored?
It appears that you can access user comments for a specific image format only. The one mentioned is for Exchangeable Image File Format (EXIF).
const CFStringRef kCGImagePropertyExifUserComment;
Take a look the CGImageProperties Reference.