I've discovered that I can use Eval for now, but if there's a better way then I'd be interested to know.
var js = @"db.ColA.find({ _id : { $in: [ 575 ] } }).forEach( function(x) {
db. ColB.insert(x);
}
});";
Database.Eval(new BsonJavaScript(js));
Cheers,
Chris.