Interesting post. Sorry I haven't got the answer (yet) but your post made me curious, because I have a similar motherboard, by MSI, with the Intel HDA also on it. Mine claims to have 'automatic jack detection' which apparently detects when stuff is plugged into the sound ports.
I don't know how they might detect if something is connected to the optical output, but:
If you go into alsamixer, do you see any slider or control in there relating to the optical port? Do you see a slider (a control) for ALL of your audio ports?
Now, if you or I were to go ahead and connect some other stuff to the audio ports, I wonder if the alsamixer panel would show anything different?
Probably not, but who knows. Either way, my interest is piqued, and if I find anything useful on the subject, I'll be sure to post it. You'll have to test it though
as I only have your standard issue desktop speaker set!