Consider an aggregation pipeline that uses $lookup to write to multiple namespaces via $out:

db.coll0.aggregate([
    {$lookup: {from: "coll1", pipeline: [{$out: "coll2"}], as: "empty"}},
    {$lookup: {from: "coll3", pipeline: [{$out: "coll4"}], as: "empty"}}
], {bypassDocumentValidation: true});

The bypassDocumentValidation specification will apply to all of the target namespaces (coll2 and coll4). If this is a feature we'd like to support, it would be nice to offer the ability to tune the bypassDocumentValidation option on a per-target collection basis.