Did you try just giving your outfile name the .wmf extension?
ggsave documentation (?ggsave) says:
> ggsave is a convenient function for saving a plot. It defaults to
> saving the last plot that you displayed, and for a default size uses
> the size of the current graphics device. It also guess the type of
> graphics device from the extension. This means the only argument you
> need to supply is the filename.
>
> ggsave currently recognises the extensions ps, tex (pictex), pdf,
> tiff, png, bmp and wmf (windows only).
>
--J