While I agree it'd be interesting to see how libuv event loop performs against the nginx event loop, keep in mind that if you're deploying a web application, nginx has a ton of other uses. For example if you need SSL termination, caching, static files served, url rewriting, rate limiting, etc., you're better off having nginx handle those then re-implementing all of that yourself. Also, unless you harden your HTTP server, you'll likely need to proxy it behind nginx anyway (as is the case with, say, tornado).
It seems to me the argument for OpenResty is: if you already have nginx in your stack, you might as well run your app in it as well.
(not saying a libuv based server in lua is pointless, I mean I've used tornado behind nginx. I also concede it would be super useful for non-web servers (say, a custom protocol/IMAP/DNS/etc.))