Thanks to hda-emu
while the kernel is compiling I discovered that to obtain
Node 0x02 [Audio Output] wcaps 0x6211: 8-Channels Digital
Converter: stream=0, channel=0
we can use against the node 0x02 the verb 0x70d with param 0x1
and to obtain
Node 0x03 [Pin Complex] wcaps 0x40738d: 8-Channels Digital Amp-Out CP
Pin-ctls: 0x40: OUT
we can use against the node 0x03 the verb 0x707 with param 0x40
so the two commands would be
hda-verb 0x02 0x70d 0x01
hda-verb 0x03 0x707 0x40
I ran this with the simulator and everything works there. I am still waiting for the AppleTV to finish compiling the kernel with CONFIG_SND_HDA_HWDEP to allow me to change these on-the-fly and try it out.
Will report the results later.