There is good reason to make this library work on non-UNIX systems. It is probably safe to ignore DOS and only worry about windows95/windowsNT portability (so filenames can be long, I think).
On the other hand, nobody should be forced to use non-UNIX systems for development.
The best solution is probably to issue guidelines for portability, like saying "don't use XYZ unless you absolutely have to". Then the Windows people will be able to do their porting.
We should also read up on what Cygnus is doing for their win32 portability.