I also use octoprint to monitor my prints ... My controller board is a Smothieboard there are plenty of spare output ports on them and its as simple as adding in a new gcode command in the config to activate them .. also my PSU is a industrial 12v 33.5A PSU which has a 5v output as well as a on off control line (just pull the line down) it would be a fairly trivial task to add a relay to deactivate the PSU thus turning everything off .. could be done remotely or by adding in a Gcode command in the completed print job scripts.

the remote plug looks like a decent solution though as long as its reliable.