Okay, so I think I've convinced myself that my deceleration noise is the same one that's been described many times before in these forums.
Like others before me, I gave the tire pressure trick a go (setting the front tire pressures higher than the rears). The first time I tried this, the noise did not go away. However, it did change: instead of starting at 3500rpm, it started at 4000rpm. With this in mind, I "tuned" the noise away by tweaking the pressures. Where I have it now (38F/34R), it seems to crop up very lightly at around 4500rpm. That's a lot better than having it scratching all the way down to 2500rpm like it was doing at the recommended 33F/32R.
Anyway, I discussed these findings with my Subaru service advisor this morning, and he was genuinely stumped by this (but he was glad that he didn't have to deal with my complaining any more). He did state that running 38F/34R was not significant enough of a difference to cause any issues with the center differential over time.
I'm glad that this simple fix somewhat resolves the problem, but I am left wondering if my car is actually a bit out of adjustment or if this is a fundamental property of the Subaru drivetrain. I somewhat suspect the latter since (1) it's a recurring issue that's been mentioned for a variety Subaru models over the past decade, (2) from what I can tell, there hasn't really been a conclusive fix for the issue (the closest being a flywheel replacement in 2002, but given that newer stock Subarus exhibit the same problem with the new flywheel, I wouldn't count that as an actual resolution).
What I can say is that I'm astounded by how sensitive the sound is to tire pressures and temperature. To me, this suggests that it's a simple but annoying resonance issue that isn't causing anything more than psychological damage. My suspicion is that this noise exists in some form in any Impreza/WRX/STI/Forester. Some people are probably lucky to have their cars tuned so it happens above redline or below idle (e.g., never), but I'm guessing that you could induce it by tweaking tire pressures.
Anyway, this is my rant/solution/conclusion. I'm obviously far from being a Subaru expert, but I think my conclusion that "it's just a Subaru noise" signifies my graduation from Subaru newbiehood