All I’d done was plug in the wrong value for the sound ID, how silly of me.
The remote setup is *finally* there. Unfortunately I have to use a pre release version of lirc for it to work, but oh well. I still had to stuff around with the lircd.conf to add scan codes for half my remote’s buttons, and then muck around with Mythtv’s lircrc file to map the keys to something sensible, but now that I know how to do it, it’s not so bad.