I'm trying to figure out why Windows won't let us do this. I think the reason is that sys.std(in|out) filehandles are not opened as pipes, and do not have the required OVERLAPPED flag set (see the CreateIoCompletionPort documentation at https://docs.microsoft.com/en-us/windows/desktop/fileio/createiocompletionport; it's that function that is used to handle pipes (via IocpProactor.recv -> IocpProactor._register_with_iocp -> overlapped.CreateIoCompletionPort).

The solution then would be to create a pipe for a stdio filehandle with the flag set.

And that's where my Windows-fu ends, and where I lack the VM and motivation to go try that out.