FINALLY, after seven weeks, I read 142 on the scale two days in a row. Plateau's over! Only one measly pound, but I'm not complaining.
A few days ago I changed my routine slightly: I added an extra TB of flax oil at bedtime. I swapped out the morning flax for walnut, so I'm now taking 3 TB total: one of flax at night, two of walnut in the morning. Not sure whether it's the extra calories or (as Talking Rat suggested long ago) that flax flavor is sneaking in through the noseclips, and separating out the walnut works better. In any case, something's working; I've got much better AS, with the constant unquenchable thirst that tells me I'm back in setpoint-reducing mode.