I just wrote this response on another topic before finding this one... I would love to have this feature!
From https://forums.plex.tv/topic/88378-external-player-option-plex-app/
I have a friend who just pointed out to me this behaviour of the DS Video app (Synology): if the app (ie iOS native player) can't play the video it asks the user if he would like to try using an external app.
The app checked if the device responded to the vlc:// custom URL scheme and, since he had the app installed, invoked it... (vlc://stream_url_here).
Plex could do the same, if the PMS could not transcode the video it could check for VLC (and/or similar apps) and simply invoke something like: vlc://http://178.162.61.272:32400/library/parts/16521/file.mkv?X-Plex-Token=czpsqLRZrAxj5Qtjs5JV
The web interface already allows direct file download, and from what I tried around on my friend's device, this solution works perfectly for folks who already have a NAS solution like a Synology but often discard Plex because their NAS isn't powerful enough to transcode videos.
My friend already uses Plex and told me he would gladly pay for Plex Pass if it provided this feature (although I think it should be available to every user). I don't know exactly how much motivation that is for you guys, that's just what he told me :)
This could bring a log of new users to the platform imho.
I have a mac mini with 10TB of external hard drives and would buy a Synology in a heartbeat if it did transcoding (the models that do 720p/1080p are crazy expensive)...
