> However, IMO, it's useful to be able to accept a connection outside of an asyncio event loop and then hand the loop the connected socket.

Looks like what you're asking for is a way to wrap existing socket object into a (transport, protocol) pair.  I'm -1 to add this new semantics to loop.create_connection, as I think it will complicate it too much.

However, we can consider adding something like 

   loop.wrap_socket(protocol_factory, sock) -> Transport