accessible_block.rb

module AccessibleBlock
  module ClassMethods

    @accessible_block = false
    def key(*arguments, &block)
      super
      attr_accessible arguments[0] if @accessible_block
    end
    def attr_accessible(*attrs)
      if block_given?
        @accessible_block = true
        yield
        @accessible_block = false
      else
        super
      end
    end

  end
end

init.rb

autoload :AccessibleBlock, 'lib/accessible_block'

sample_model.rb

class Model
  include MongoMapper::Document
  plugin AccessibleBlock

  attr_accessible do
    key :somekey
    key :another key
  end
  key :private_key
end

Might want to call super before attr accessible. I would have to check but i think it is relatively important that key returns the key.