I guess you might write a filter which dives into the media bag to get the wmf images, calls a command line conversion program through a pipe, either with writing to/reading from intermediate image files or piping the image data directly, then puts the svg images into the media bag and adjusts the src attribute of the image element appropriately. I'm afraid I can't help you with the details, not even what conversion program to use, as I never have done that particular conversion, nor touched the media bag. You can read about the media bag here:
about the pandoc lua module's pipe function here:
and about Lua's piping mechanism here:
and about Lua's mechanism for executing external commands here: