I actually made a post about this a while back, there's nothing I can do on my end to make hoverzoom support my site, I've emailed them and asked them to include it in their next release but I haven't heard anything back. If you and other users want to email them too then hopefully it will be included in the next release. If you can't wait Imagus is a hoverzoom alternative that will support slimgur. I write more about it here: https://voat.co/v/slimgur/comments/182122