Hallelujah! Oh man, I can’t believe that worked! After years of struggling, I can finally force files to sync without needless transcoding! The party line from Plex has always been, “Plex knows best” and “Plex is made to transcode everything and if you’re not willing to do that you’re an idiot and shouldn’t be using Plex”. Not a direct quote but close enough. 
I’m on a low-power Synology DiskStation which should never even attempt to transcode. If it helps anybody, the path for this profile XML is:
/Plex/Library/Application Support/Plex Media Server/Profiles
I added mp4 to the VideoProfile containers since that’s how I roll.
Next up: is it possible to use custom device profiles to make the photo viewer stop dynamically resizing all images before sending to the device?
