as i said before, you can use two files (one for input, another for output) as the transport, i.e., you can write your own FileStream class that takes two files, where read() reads from the input file and write() writes to the output file. of course the two ends should connect to opposite files, e.g., if the client uses /tmp/f1 for input and /tmp/f2 for output, the server should use /tmp/f1 for output and /tmp/f2 for output. other than that, it should all work the usual way. note that there's no real "server" in this case, it's only two processes that communicate over these files (there's no need to "connect", only open a file).
you an also use pipes, of course. the PipeStream classes also uses two pipe objects (one for input, another for output). i can refer you to https://github.com/tomerfiliba/rpyc/blob/master/rpyc/core/stream.py#L139 and to https://github.com/tomerfiliba/rpyc/blob/master/rpyc/scripts/rpyc_classic.py#L151 -tomer
