For the life of me I can’t find the project where I had set this up before, and the colleague that helped me with the material is out until next week. However, I do have a second method that would produce better results than what that would have anyway.
So with DitheredTemporalAA there is a way to do it from what I recall that would require a sine node at a high enough rate that it was able to shadow, but for your needs you could easily get away with just using a second skeletal mesh and setting it to not be visible. Then under the lighting tab you can set hidden shadow to true. Since the second mesh will not be visibly rendered you can use a lower poly version if you need, since you would only care about the shadows being cast.