How to render and set custom mipmaps for THREE.WebGLRenderTarget

Question

I'd like to generate custom mip maps for a rendered frame for an effect in THREE.js, but it's not clear how to achieve that. I'll want to regenerate the mip maps again after every frame.

This example shows how to generate mipmaps for a texture, but it looks like the mipmaps array requires a handle to an image or canvas tag, which a renderTarget texture does not have. Ideally I'd like to be able to do something like this:

var rt = new THREE.WebGLRenderTarget()

// initialize and render to the target

var mipmaps = [];
var width = rt.width / 2;
var height = rt.height / 2;

while( width > 2 && height > 2) {

    var mip = new THREE.WebGLRenderTarget({ width, height });
    mipmaps.push(mip.texture); // mip.texture.image === undefined

    // render to the new mipmap with a custom downsample shader

    width /= 2;
    height /= 2;
}

rt.texture.mipmaps = mipmaps;

// render using the original render target as a texture map and using
// the new mip maps as needed

Is this possible? How would I achieve this?

Thank you!