I created a free tool to do this here: https://nixnoob.com/svg_webp I also created a short post about it that includes a sample svg. The original png version is 6.12 MB and the WebP compressed version is 340 KB.

One thing to note is that Inkscape does not support embedded webp in svg so if you open an svg that has embedded webp images in Inkscape you will see the image placeholder that has a red circle with an X in the middle. You can still edit the svg and save and the embedded webp will still work in the browser.

I am thinking of adding a tool to reverse the process as well.