Alright, beta 7 is now out as well with the beginnings of the new Retroarch Netplay integration (largely thanks to @Izegod who provided a plugin that did it a while ago). If you right-click on a game that supports Retroarch in LaunchBox, you should have a new Retroarch Netplay option. I will be integrating it with Big Box as well, though I haven't started on that yet.
I need everyone's help in testing it though, as Retroarch netplay is notoriously difficult and finicky. I'm currently copying stuff over to another machine of mine to give it a proper test. However, I could not get it working over localhost (I end up with a "netplay could not be initialized" message in Retroarch).
@Typhon I was actually referring to something else above, but I did get that fixed in this beta, so we should be good to go. It worked for Alien3, at least.