Seems to me the problem here is that mingw is passing these paths into Wine at all. The only apparent way to get to DOSFS_GetPathDrive is through DOSFS_GetFullName or DOSFS_GetShortPathName so either mingw is calling one of those functions with Unix path names or its calling some other Win32 API function with Unix path names which then calls one of those functions. But how would that work on Windows? Surely mingw should always convert unix paths to Windows paths *before* giving them to any Win32 function?
I have a feeling the Windows path handling routines are very flexible, and will accept unix style paths, I know you can use / instead of \ in paths and things usually still work. I guess some test cases would be needed.
David