For what it's worth, here is some example test code I wrote for win32 demoing overlapped serial I/O: http://www.alumni.caltech.edu/~dank/serio/serio.zip It might be of some use to people testing wine. I haven't tried it yet there.
Also, http://www.alumni.caltech.edu/~dank/overlap.htm chronicles the problems many people have had with serial I/O under win95. The messages there get into interesting details of how people use overlapped serial I/O, and how serial I/O really behaves under various flavors of windows. Again, might be of some use, who knows. - Dan