> when converting from jpg to webp, XMP and exif metadata is lost.
https://crbug.com/webp/166 deals with an imbalance between the library based (libpng, libjpeg, etc) and native windows importer (WIC = Windows Image Component). You can try the -nowic binaries  as a workaround.
For the question in the subject we never defined an IPTC chunk for the format  -- I don't remember if there was a reason or it was just an oversight --, but it could be added to a file as the chunks are extensible. This would require some custom file manipulation currently as webpmux doesn't support adding arbitrary chunks to a file .
IPTC was not really widespread at the time the chunks were defined. Moreover, it's been recommended since to embed IPTC data into XMP instead of using the separate IPTC metadata format, which is loosely supported.
I do not consider myself competent enough to write a custom RIFF container for IPTC metadata in a webp file. I have decided to put my IPTC metadata within XMP tags for the moment. maybe one day IPTC will be standardised for webp.