It seems that this problem can be solved by making a .thumbnailer entry (similar to a .desktop entry) in /usr/share/thumbnailers as described here: https://askubuntu.com/questions/617047/how-to-preview-dds-and-webp-images-on-nautilus Maybe the .thumbnailer entry should be included in a +gnome useflag for libwebp. It also might be worthwhile to do this with imagemagick as well, considering that it supports more file formats.
I think this got resolved a long time ago. It works now.