Instructions for configuring MythTV to wake the PC when a recording is scheduled can be found several places, including here. Following a similar but slightly different set of instructions (whose location I've since forgotten), I tested my BIOS, created the setwakeup.sh script and tested that, configured both the back end and mythwelcome, and then scheduled some test recordings. The setwakeup.sh script worked fine when run (with administrator privileges) from a command prompt, but I found that the PC woke up at times it was not supposed to, and failed to wake up when recordings were scheduled.
Long story short, the key was in the mythwelcome setup (accessed from a terminal via mythwelcome --setup). There is a field labeled nvram-wakeup Restart Command, and for some reason it was populated by default. The instructions on the MythTV wiki state that this must be blank, and they're not kidding. Clearing this field at least partially fixed the wake up issues. MythTV now wakes the PC (from an S5 state, i.e., powered off) via a BIOS alarm and successfully handles scheduled recordings.
Two wakeup issues remain:
- I find the PC booted and running at times when nothing is scheduled (and I have no indication why it powered up); and
- the back end is supposed to wake the machine between 0200 and 0500 local time to download program listings, but that appears not to be happening, at least not regularly.