BTW: there is a patch here sdlmess-coco-dwsck-ffe0.patch
This patch opens a socket (inet) connection and hooks it directly to the address bus of the CoCo. Obviously something that would never make mainline.
I'm told the implementation works on Mac, Linux and WIndows. Maybe this could also be fit into sdlfile.c. Either in addition to the pseudo terminal, or as a replacement to the pseudo terminal.