dataset.rb

module Sequel
  class Dataset
    alias_method :execute_orig, :execute

    def execute(*args, &block)
      SequelNotifications.subscribers.each do |subscriber|
        subscriber.call(*args, &block)
      end
      execute_orig(*args, &block)
    end
  end
end

sequel_notifications.rb

class SequelNotifications
  class << self
    attr_accessor :subscribers
  end

  @subscribers = []

  def self.subscribe(&callback)
    @subscribers << callback
  end
end