Lead Developer
Hmm, the above code definitely wouldn’t do it.
You may be able to remove the current post thumbnail with a remove_action call, and then insert your own post thumbnail function in with included code for the caption using a plugin like this: https://wordpress.org/plugins/featured-image-caption/
However, it would involve a decent amount of PHP to do so.
I can look into it further if you like, but I think a or b would still work, and may be better with less hacking into the core theme.