Thanks for figuring this out, John. I am thinking it might be easier for ordinary folks to add the code for the featured RSS via a plugin?? It’s something I find confuses people in a syndicated class, because they added their image to WordPress, but featured images exist outside of the RSS.
If you don’t figure it out, I might 😉
Oh never mind, it’s been done!
https://wordpress.org/plugins/featured-image-in-rss-feed/
It wont help my current group of students because they are in WP.com[promise]