Hey guys! Sorry for the delay. Yes, they do appear to have fixed it! The overall behavior now is still a little different than the day I bought it, but the fluctuation is gone. Now, my needle warms up in bursts... I'll see it make brisk moves of about 1/4th a notch every few seconds, until it reaches about 95% of normal. Then, it will stay at about 95% for a good 5-10 minutes... why, I don't know... until it reaches 100% of normal. Once there, it stays there completely. I haven't seen one movement at all.
The reasons for the not-perfectly-linear warm-up cycle could be many. I've had several coolant flushes, a few thermostat replacements, and my current coolant has come from a tuner shop and may not be exactly what Subaru's computers are expecting. (I thought maybe a third party could have found something Subaru didn't....) But the fluctuations are gone.
My service paper says the following (Paraphrased) "Performed wiggle test on the main engine harness while observing ECT sensor and was able to duplicate the fluctuation. Technician determined the issue to be ECT signal wire connector B21 & E2, Pin 12 disassembled, connectors checked for sliding resistance and resoldered and crimped. Retest shows ECT now steady."