Something like this should work

defmodule MyApp.Endpoint do
  use Plug.Router, copy_opts_to_assign: :builder_opts

  plug(:match)
  plug(:dispatch)

  def dispatch(conn, _) do
    super(conn, conn.assigns.builder_opts)
  end

Thank you very much!
It seems to work now. Even if it’s hard to understand how you get to this solution.

The new option makes builder opts available as an assign (on conn.assigns) with the given key. Then you need to pull out that value for that key to pass to dispatch (as before). You can either do that by wrapping dispatch in a custom plug or given dispatch/2 is defoverridable you can implement a custom function for it, which delegates to the generated one using super.