How to use custom shadow models?

How can one use model A for the shadow pass and model B for the color pass?

In past versions of Unity a trick could be used to change LODs in between the shadow and color passes. Unity 4.5 stomped this. (Screenshot of old Unity)

[32545-fast+shadows.jpg|32545]

Have a child shadowcaster object with the renderer set to Shadows Only (with the simpler shadow mesh), and set the main Renderer to not cast shadows.

This is more 2020 than 4.5, but ¯\(ツ)