Plex Live TV & DVR documentation. The FAQ lists supported clients. LG WebOS is supported.
Plex Online Media Sources documentation. Free Live TV Overview lists supported clients. LG not listed.
You may want to subscribe to the Plex for webOS announcement thread. It is updated when new releases of the Plex client are available and lists new features, fixes, etc. To subscribe, scroll to the bottom of the thread and change Normal to Watching.
