Selective sampling

Let's say I have a large render that's mostly clean, like the following one:


Where most of the noise is concentrated in a small area of the image because of the high density of emitters compared to the rest of the scene. It will probably take another 10-40k SPP to clean that noise up. At 1920x1080, that could take a while, considering the given render is currently at 13k SPP after about five hours.

Would it be possible to select only that area of the image to continue rendering?

If I understand path tracing correctly, the sampling is based on pixels, so it should be as simple as bounding further sampling to the selected area.

With a reduced sampling area, it should be as clean as the rest of the image in a lot less time than it would have taken sampling the image as a whole, with the render time shortened roughly proportionally by how much smaller the selected area is compared to the canvas.