Reindex w/field collapsing

[F_Doe_FW (Frederick Doe)]

Is there any way to collapse results with ReIndexing? I have a source index and want to get the most used pieces out, and only return their 'natural' key into a new index.
I can't figure out how to stuff the collapse logic into the Reindex process... query? aggregation?

    {
      "max_docs": 1,
      "source": {
      "index": "toplanes",
    		"_source": ["compkey"],
    		"collapse":{"field":"compkey"}
    	},
    	
      "dest": {
        "index": "hot_lane_lookup"
      }
    }