You may want to consider using Posix aio to implement overlapped I/O, regardless of whether it's a disk file, a network connection, or whatever. glibc has a fallback implementation of aio, and vendors are starting to provide optimized, high-quality implementations. SGI and Red Hat are working hard on it.
I had a look at it, but the current implementation on my system (libc 2.2.2) is limited to files that allow lseek() and therefore only suited for disk files. Moreover, it requires pthreads. I didn't feel bold enough to check whether it works with Wine's pthreads replacement (btw, aio internally uses threads for doing IO, so if threads are the wrong thing, as Alexandre argued, aio isn't right either).
Anyway, thanks for the hint, I'll stay tuned with aio.
Martin