Aha.
Finally had a chance to do some deeper digging around in the support forum for the card and found that a lot of other people were asking the same question about FP audio for headphones.
Apparently (at least according to the most helpful reply I found so far) the card simply does not have auto-detect for the FP headphone jack and no matter what internal connections you set up it will never disable the speakers and switch output to 'phones just because you plug them in there. There is indeed some type of option in the audio control software that has to be switched over by the user. Can't understand why they would give the option of FP input at all and then not put in an auto-detect feature, but what do I know?
*shrugs*
I could've sworn I clicked on something in there that said "Headphones" and it didn't work but maybe I had the FP cable plugged into the motherboard at that point and that made a difference, who knows? I've forgotten how many different combinations of things I toyed with, lol.
Anyhoo - since the speaker system jack *does* have auto-detect, that seems like the best way to go all around.