I said I would provide an update if I saw any signs of recovery. Here's what the whole sorry affair looks like right now:
Despite the catastrophic die-off, there are definite signs of life. The Columbus (closest to the camera) definitely has some green leaves left. The contrast between the dry and dead leaves with those still alive is pretty striking. Next along is the Cascade, also with some signs of life lower down. It's harder to see with the Willamette at the end but there are some leaves still maintaining their form. All three also have at least one small shoot that I'm going to tell myself is new growth. Others may disagree. This is the Columbus:
Growing buds like this are nearly always the first to die when the plant suffers severe damage, so I have high hopes here.
Here's the Cascade:
If you look very closely you can see a small bud beginning to emerge to the left of the main stalk.
The Willamette also looks like it might have some new growth:
The main stalk to the right looks to have two new shoots, which are of course slightly out of focus. This picture also shows how some of these leaves are only just (but bravely) hanging on, having been eaten by bugs and also discoloured.
Overall I'm quietly confident now that they're not going to completely die off. The biggest concern I have now is whether they will be able to recover enough, before the cold weather appears, so that they can survive the winter and appear again in the spring. Fingers crossed as usual.