Not sure why this wasn't done yet. It is so trivial to do this that a very naive implementation doesn't even take more than 9 lines:
$(".flow-reply-link").parent().append('<a class="flowviewsource"> View source </a>');
$(".flowviewsource").on("click", { type: "normal"}, function (){
flowPost = $(this).parents(".flow-post-meta").siblings(".flow-post-content")[0].innerHTML;
$.post("https://en.wikipedia.org/api/rest_v1/transform/html/to/wikitext", {
html: flowPost,
}).then(function (data) {
alert(data);
});
});
It can probably be polished up and made into a gadget if the developers aren't interested.