I realize this is a bit old now but just throwing in my 2 cents. On the http input node I set it to POST and Accept File Uploads (this feature may have been added since the original post), I pass this into a function node with these two lines in it (very similar to how iura.alin) . . .
msg.payload = JSON.parse(msg.payload.payload);
Also @groovygrovesy 's the url’s in node-red are relative to your base url, if you access node-red via http://localhost:21000/red and create a node called /plex then it’s full url is http://localhost:21000/red/plex I realize your issue is resolved but just in case it wasn’t clear to other people who find this post.
I would be very interested to know what people have come up with, I saw the suggestion on the Plex website about dimming the lights when you play a movie, sounds great, anyone got a IoT popcorn maker?