It’s on my feature request list already, promise :). I have real major concerns about even attempting to add it because I do have many users which have hundreds of thousands of entries and such a system is not scalable to those numbers, easily. When I do add this, I will first test to see how Yoast handles it… I suspect Yoast simply does not include that many posts/pages in the sitemap or worse, just crashes because the server does not have enough resources to handle it. It’ll be interesting to learn.
Like I’ve mentioned Google will index the entries just fine, I’ve gone thru great lengths to ensure it does and that is what is truly important. By far the most important thing is that your site has quality content, kept fresh as that is the most important factor for your site rank in the index. If you do not have that, it does not matter what the sitemap contains or does not contain.