Overpass API: find relations that completely encompass map area

I have an application that retrieves small areas of map data via the Overpass API to generate custom-rendered maps of interest. However, if the area of interest is completely inside certain relations (e.g. in one case, a multipolygon of natural = heath), this relation is not returned by the bbox query and therefore the landuse is not tagged appropriately. I understand that simple areas (closed ways) and named relations can be retrieved using an "is_in" query - but how can I retrieve unnamed relations such as the example above? Thanks!