Uh, holy crap??? That's amazing. I hadn't even thought of trying something like that. That's brilliant and ingenious. At this point I'd just resigned myself to transcoding where needed (usually just need to change the container, which takes one ffmpeg command, and about 2 minutes for a one hour video) and let iTunes handle everything, but I have a major soft spot for Plex and miss using it. That's just phenomenal. gushing.stop();
Another interesting option is Beamer[1]. Bought it a while ago and I have nothing but good things to say about it... Just drag any video on it and stream it to your AppleTV in seconds.