A rough solution to https://github.com/mongoid/mongoid/issues/3794

mongoid_embedded_object.rb

module MongoidEmbeddedObjectModification

  # Custom field behaves more like an immutable value.
  # Your changes to the object obtained with getter method will not take an effect unless you assign it back.
  # https://github.com/mongoid/mongoid/issues/3341
  def modify(mongoid_doc, field_name)
    obj = mongoid_doc.public_send(field_name)
    yield obj
    mongoid_doc.public_send("#{field_name}=", obj)
  end

end

# ================

module MongoidEmbeddedObject
  extend ActiveSupport::Concern
  extend MongoidEmbeddedObjectModification

  def self.new_class(&block)
    Class.new do
      include MongoidEmbeddedObject
      instance_exec(&block)
    end
  end

  module Mongoization
    extend ActiveSupport::Concern

    module ClassMethods

      def demongoize(object)
        self.instantiate(object)
      end
      
      def mongoize(object)
        object.mongoize
      end

      def evolve(object)
        mongoize(object)
      end

    end

    def mongoize
      attributes.except('_id')
    end

  end

  # HACK: Mongoid::Inspectable includes hardcoded _id field
  module InspectableWithoutId
    include Mongoid::Inspectable

    def inspect
      super.sub('_id: , ', '')
    end

  end

  included do
    include Mongoid::Attributes
    include Mongoid::Changeable
    include Mongoid::Fields
    include Mongoid::Threaded::Lifecycle

    include MongoidEmbeddedObject::InspectableWithoutId
    include MongoidEmbeddedObject::Mongoization
    fields.delete('_id') # Mongoid::Fields adds '_id' field; for now, its accessor methods are not removed
  end

  module ClassMethods

    def instantiate(attributes = {})
      self.new do
        if !attributes.is_a?(Hash)
          raise ArgumentError
        elsif attributes.present?
          @attributes = attributes.deep_dup
        end
      end
    end

  end

  def initialize(params = {})
    @attributes ||= {}
    apply_defaults
    if params.present?
      params.each do |attr, value|
        self.public_send("#{attr}=", value)
      end
    end
    instance_eval(&Proc.new) if block_given?
    super()
  end

  # HACK: method in Mongoid::Document needed by included Mongoid modules
  def __selected_fields
    nil
  end

  # HACK: method in Mongoid::Document needed by included Mongoid modules
  def new_record?
    true
  end

end

Please see also the MongoidCustomArray class that should go well with this:
https://gist.github.com/ippeiukai/0dc7ec74abe62a599394