Suneido doesn’t haven’t anything for this (yet). I see on Windows zero-copy is done with the TransmitFile function which needs a file handle an a socket handle.
Currently, if you use Suneido’s built-in File and SocketClient classes, there is no way to get the Windows handles, although this could probably be added.
Another option would be to use the Windows api to open the file and socket so you would have the handles. There is a wrapper in stdlib for the Windows api CreateFile function, but I don’t think there is anything similar for sockets, although it probably wouldn’t be hard to add.
I wonder how much difference it would make. Network is slow, relatively. It would put less load on the cpu, but these days we usually have lots of cpu to spare.