Currently parseBasicAuth is not exported and used only by BasicAuth function which always uses "Authorization" header. Proxy server can use something like "Proxy-Authorization" or "XXX-Proxy-Authorization" headers and currently it's not possible to reuse parseBasicAuth to parse basic auth payload there. My proposal would be to simply export this function so proxy servers could apply it like:

    auth := r.Header.Get("XXX-Proxy-Authorization")
    if auth == "" {
        http.Error(w, "", http.StatusProxyAuthRequired)
        return
    } 
    username, password, ok := http.ParseBasicAuth(auth)