omit() will remove any fields specified from the message and then send down the pipeline. It currently only works for top level fields (i.e. address.street would not work).
omit({"fields": ["name"]})message in
{ "_id": 0, "name": "transporter", "type": "function" }config
omit({"fields":["type"]})message out
{ "_id": 0, "name": "transporter" }