In a way, Terragen can do this with the adaptive sampler. The adaptive sampler interpolates samples, and can interpolate samples that have a spacing of more than a pixel if the "min samples per pixel" is less than 1. It won't be as fast as rendering a lower resolution image, but is much faster than fully sampling every pixel. The Robust Adaptive Sampler is usually better than the Legacy Adaptive Sampler for this purpose.
Of course, if there are large enough differences between adjacent samples, the adaptive sampler will take more samples instead of interpolating. The pixel noise threshold defines what "large enough" means. To encourage the sampler to interpolate instead of taking more samples (to simulate a low resolution render) you need to reduce the noise in the sky, so you'll need to increase the atmosphere and cloud quality settings to the point where they don't produce much noise between adjacent samples. But, if you think about it, you'd need to get your pixels to be fairly noise-free in a low resolution render too, otherwise it would look blotchy when you upscale it. Whichever approach you take, the cloud and atmosphere need to be sampled well enough to avoid excessive noise.
