Going back to my apprentice days as a boatbuilder/shipwright with this discussion! The purpose of spiling at full scale is to figure out how to cut a plank from flat stock that can be bent around a curved shape without signficant edge-setting (in geometrical terms, the ideal is to bend the plank in one axis only, with some twist allowable). For this to work, the spiling batten needs to resist edge-setting as much as possible, so we often used thin plywood for this. It allowed us to take the shape of the plank off the frames and planking already erected accurately, so we could cut the next plank accurately, so in theory, one could make planking out of plywood as well. The main limitation is not its lack of flexibility (that is actually desirable) but that once in place, fairing the surface tends to go through the outermost layer of veneer and expose a lot of end grain, which is a rot starter. Plywood is mostly used as planking on boats with hard chines at the seams.
I think making "spiled" planks as kit parts is problemmatical, since the point of spiling is to derive the flat shape of a plank from the faired shape of the frames. If the faired frames deviate from the theoretical shape even a little, planks cut to patterns will not fit and may be more work to correct, which then makes them narrower than planned, with a cumulative effect. But if the hull is properly faired and the plank is lined (laid out) correctly, then it should be possible to make individual spiled planks from relatively narrow flat stock without a lot of short grain, although the effect of working at smaller scales might be challenging. At full size, we tried to get the spiling correct to within 1/16" (1.5mm) on boats and no more than 1/8" on larger vessels to make shaping go quickly. These kinds of tolerances are not achievable at scale, and I suspect most people would not want to work with scale width planks at smaller scales. But there is nothing to stop you from making spiled planks in larger scales, or from spiling planks that are wider than scale as a concession to size, and it should be possible to do it from normal sheet stock of moderate width.
As an historical curiosity, the Beetle Cat, a small catboat made in large numbers by the Concordia Boat Company, used planking patterns. The builder, Leo Telesmanick, developed the system to streamline production. By the end of his career, the patterns had become so worn from being traced onto generations of planking stock that Leo knew how much to allow outside the pencil line for a good fit. He had also committed to muscle memory the rolling bevels along the plank edges, and could often make them with a single pass of a bench plane. But he was building boats on a rigid form from mass-produced components for the frames, so the variation in shape from one boat to the next was minimal.
Fred