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How about we use connect_socket() or a variant on that name? That feels similar to connect_{read,write}_pipe(), which also take a protocol_factory and an already-opened I/O object.
If it's only for server-side sockets I'd call it connect_server_side_socket() -- I don't care that the name is long, the use case is not that common. Or we could have connect_socket() with a server_side flag and a server_hostname argument, and refactor some things so that it shares most of its implementation with _create_connection_transport().