Domain fronting - how?

I believe what I want to do is referred to as "domain fronting". I want to serve a subdirectory of a site as the root of another. See this post.

I see "fronting" mentioned a couple times in the documentation, but it's just in passing. There's no explanation about how exactly to implement it.

I have successfully "mapped" one domain onto another, and that's working fine so far. However, when I try to use PathPrefix() or replacePath, it always leads to a 404.

Is there a way to modify the HTTP/2.0 :path pseudo header directly? Anyone have any insights to share?