I have a similar use case where I'm trying to do this. AEM allows you to overlay metadata for Content Fragments and by default CFs allow content authors to add tags to their content. I need to overlay the search facet in the Page Editor for Content Fragment search, to add tags as well as other custom metadata predicates.
I can easily overlay the Page Editor (Image search) but I can't find a way to overlay Page Editor (Content Fragment search). Is this possible?