For posterity, this works…

{:ok, pid} = Plug.Adapters.Cowboy.http(MyPlug, [], ip: {:local, "my_plug.sock"}, port: 0)

The port: 0 is important; it won’t work without it.

Also, you gotta rm my_plug.sock when you’re done otherwise it won’t start up again: :eaddrinuse (address already in use)

With :gen_tcp you can use the tuple {:local, path} to connect to a unix socket. It is mentioned in the :inet module docs:

If you want to make HTTP requests to a unix socket you can use HTTPoison/Hackney with the “http+unix://” scheme, which is a bit of a hidden feature:
https://github.com/benoitc/hackney/blob/master/src/hackney_url.erl#L45

Turns out the new file system in OTP 21 fixed the issue, so Phoenix should now run over a unix socket.