So this may or may not work for everyone, but I have a low-tech method of keeping my system off when not streaming.
Use something like a Wemo power plug for your system. You can turn on your system remotely (just configure the computer’s bios to turn on when it gets A/C power to the PSU). Stream to your heart’s content.
The plex server is at least smart enough to keep the system from sleeping while streaming. When you’re done streaming, it’ll go into sleep mode. (if your system is configured to do so)
