Again, thanks. Let me give a little more detail.
Over my base layer, I have Layer 2, at about 50% opacity, with some image info on it. It has a layer mask, substantially restricting its effect. I then have a HSL adjustment layer, which i want to desaturate only Layer 2. The overall effect of Layer 2 is to put some detail into a blown-out highlight area in the base layer.
The little icon in the adjustment layer's properties, which Christoph mentioned, seems to do exactly what I was looking for. If there are other approaches, I'd be interested in those too. The "clipping mask" approach seems inflexible, since it doesn't adjust very readily if I edit Layer 2's mask.