Forward rendering:
- Most of "shadow sampling" is in AutoLight.cginc include file somewhere in Unity.
- Now, if you copy that file into your project somewhere, and have some shaders in the same folder - then these shaders will use yur local "hijacked" copy.
- In all the functions that sample shadowmap, you could for example add more taps etc.
Deferred lighting:
- most of shadow sampling code is in Internal-PrePassCollectShadows.shader / Internal-PrePassLighting.shader.
- copy these built-in shaders into Resources folder of your project (Resources so they are always included into game build)
- restart Unity and hack them.