console.log

Set mixed { '0': { name: 'Foo' },
  '1': 
   { path: 'mixed',
     instance: undefined,
     validators: [],
     setters: [ [Function] ],
     getters: [],
     options: { type: [Function: Mixed] },
     _index: null } }
Set mixed { '0': 'Bar',
  '1': 
   { path: 'mixed',
     instance: undefined,
     validators: [],
     setters: [ [Function] ],
     getters: [],
     options: { type: [Function: Mixed] },
     _index: null } }

mixed_type_setter.js

var mongoose = require('mongoose');

var schema = new mongoose.Schema({
    mixed: {type: mongoose.Schema.Types.Mixed}
});
schema.path('mixed').set(function (data) {
    console.log("Set mixed", arguments);
    return data;
});

var TestModel = mongoose.model('TestModel', schema);

var obj = new TestModel({mixed: { name: 'Foo'}});
obj.set('mixed.name', 'Bar');