You could write your own shader (based on one of the shaders provided in the spine-unity runtime) that adds such a gradient overlay effect at the end of the fragment shader. This would then render in a single pass and save resources.
If you for whatever reason need to keep the overlay separate: Alternatively you could write a (or use an existing) gradient shader that renders the existing skeleton mesh again. You could use the RenderExistingMesh component to re-render the same skeleton with a different Material and shader. This however comes with the drawback of multiple semi-transparent pixels of overlapping attachments being overlaid multiple times, so here we would recommend having z-write and z-test enabled to avoid such multiple overlays. For these reasons we would recommend the first solution over this one, unless you don't care about overlapping attachments.
