I'm sorry to resuscitate this topic, but I've recently tried to use Affinity Designer for a Pixel Art project again, and it feels like there are still issues preventing this feature from being used.
I'm using coverage maps to create a sharp transition, which seems to work just fine... until you start overlapping objects. In this example here, the brown shape is nested inside the dark green shape, and whenever their edges "touch", antialiased pixels are created. I've also set view quality to nearest neighbor, but no luck.
Affinity Designer would be so awesome for this kind of task, specially because it so seamlessly blends pixel and vector workflows. But for now, the worlds of vectors and pixels shall remain separated, it looks like. :-(
Leo.