From libreoffice's man:
OPTIONS
--accept=accept-string
Specify a UNO connect-string to create a UNO acceptor through which other programs can connect to access the API.
So open/libreoffice uses this approach to allow other programs and even client machines to run the program.
My suggestion: forget the complexity and run the following command from your program:
libreoffice --headless --convert-to png /path/to/file.wmfORemf
Here is a link to post describing how to run a command from Java