custom light calculation / shadow projection. possibly SRP?

i want to modify the way my main direction light works:

- i want to update the shadow Depth texture only once, when the Scene is loaded.

-i want to change the shadow projection Matrix to match a voxel grid.
meaning i want the z axis (depth, in shadow space / light direction) to stay the same, but modify the x and y axis (xy on the shadow Depth map) to match the Unity x and z axis.
furhtermore, i would like to modify the scale of These axes so each Pixel in the shadow map represents one voxel.
see Image (left is Default, Right is what i want)


it seems to me like this was something the new SRP could do, but i can not find a starting Point on how to modify light calculation.

on the other Hand i was thinking About using a custom script which renders the custom shadow map and passes it to a custom material with a custom shader. but this would be far less flexible when adding more Lights etc.