WebMock overrides the Net::HTTP#request method to do it's thing, but Net::HTTP::Pipeline circumvents that method to do *it's* thing. We only care about the responses during testing, so getting WebMock behaviour for pipelined requests is most easily done by just not pipelining them.

webmock-net-http-pipeline.rb

WebMock::HttpLibAdapters::NetHttpAdapter.instance_variable_get(:@webMockNetHTTP).class_eval do
  def pipeline(requests)
    responses = []
    requests.reject! do |req|
      response = request(req)
      yield response if block_given?
      responses << response
    end
    responses
  end
end