After a lot of pain I managed to setup acpi wake up for mythtv. I set the computer to boot at a certain time, autologin and start recording. I tested it and it does work.
Now, I'd like to make it automatically shutdown when recording is over, but I don't know how to achieve this.
I followed the guide at http://www.mythtv.org/wiki/ACPI_Wakeup with some arrangements (used rtcwake shell program). The checklogin.sh script is the very same, indeed.
No auto start mythfrontend at startup
ACPI Wake from state S5 (soft off)
Autologin at startup
Ubuntu 12.04 x86_64