They already do it for direct play of files, as long as you are playing the video in their native Plex Media Player.
Since they have and use an HDR to SDR tone map solution for direct play, one would think they would transcode to 10bit/bt2020 and just change the resolution. Then they could apply the same tone mapping they use for the native video.
HDR video played on SDR monitor using Plex Media Player native tone mapping through direct play
HDR video played on SDR monitor using Plex Media Player after transcoding.
