Having got ArchLinux up and running on the TonidoPlug 2 it was a trivial matter in the extreme to get a functioning MythTV backed up and running. Installing the packages was straightforward and simply done via the standard package manager - pacman.
The only minor problem with the setup and configuration is how on earth you run the setup without a display. Thankfully it's easy and you can simply rely on X11 forwarding.
Recording a single SD show results in an ~30% CPU usage load, which I am more than happy with for now. I've not ventured into trying to record HD signals yet so the plug may not be up to the job, but we shall see.