Is there any merit in having someone stick their ear by the bike to "listen" while I pedal? Like you guys said, sound location can be "deceiving."
Can be. Creaks can be a pain to isolate. I had one and my mechanic and I couldn't find it, so on a group ride later that week we rode side by side and he asked me to try and recreate it. In my case it turned out to be the spokes all things. Where the spokes cross on the rear wheel some had a little grit in there, possibly also a very slight but mostly unnoticable groove or tendency to wish to stay at one location. When they moved under some load, a little creak occurred. I had tried everything too. Thise was fixed pretty much premanently with just a dash of lube at the crossing points.
Mostly just go through the likely suspects. You'll often find it this way. Check, clean and re-lube (or put new carbon assembly paste in, depending on what we're talking about) skewers, seat post, and seat post bolts. Check the tension of the crank bolts and possibly remove and re-lube those. Most times I've found it to just be one of these things.