Ok, this worked, with some additional config. For me to add both a path picker and a tag selector (to bring it in line with what is there for Images), I had to overlay a few things:
1. Overlay the search panel items to add a new new path picker
Path: /apps/wcm/core/content/editor/jcr:content/sidepanels/edit/items/tabs/items/assetsTab/items/filterPanel/items/search/items/searchpanel/items
2. Overlay the search panel 2 items to add a new path content fragment facet
Path: /apps/wcm/core/content/editor/jcr:content/sidepanels/edit/items/tabs/items/assetsTab/items/filterPanel/items/search/items/searchpanel2/items
3. Overlay the search facets to add a new content fragment tag predicate facet
Path:
/conf/global/settings/cq/search/facets/contentfragmentsThanks for your help!