If you know a little php programming and .htaccess routing you can make a simple php script that handles the robots.txt contents according to your own rules: Time of day, IP address, etc... Just a thought, i thought it might interest someone.
the problem that the bots check the robots file only so often, not like every hour to see if you changed it so your idea may either don't work or backfire when google bot would check your robots file during certain times of the day and it would always tell it "don't crawl"