Tim, I would like to ask similar question because you know the problem.
I am also using the stained glass window and I would like to achieve good quality shadows for the “metal framing” of the glass. I do use three planes for each window (with translucent material, emmisive colored shadow material, and metal framing masked material) and two directional lights in the scene, one light is static for the colored shadows, and other is stationary for proper cascade shadow. I have a problem with that cascaded shadow because it is defined by the distance from the camera, and with several windows I am able only to adjust the cascaded shadow distance to work only with one of them.
as shown on the screenshot, first window is having decent looking shadow, and the one further from the camera is not having any.
