The underlying library of guacamole-server, libguac, has historically been the most portable part of the stack. Unlike guacd, which relies heavily on POSIX, libguac relies primarily on standard C99, and should up to recently could be compiled on Windows. Recent changes have resulted in libguac failing to build on Windows platforms, but those changes are minimal, and alternative/additional approaches for the same should bring things back to normal.
For the sake of allowing Windows applications to be built which leverage the Guacamole protocol (an application which streams itself, the Windows equivalent of the X.Org driver, etc.), this portability should be restored.