Graphics.Blit into lower MipLevel?

What is the cleanest way currently in Unity to render a "full-screen" pass with a custom shader into a miplevel of a RenderTexture? There is no option on Graphics.Blit itself, so it looks like you would have to use Graphics.SetRenderTarget in some way (since it has an overload that accepts a miplevel). But then how would you go about actually rendering into it using a fullscreen pass and custom shader/material?