I just stumbled upon this nifty site. It creates a pop up on your site showing how much cheaper it is to book direct - it gives rate comparisons with big OTA's. http://www.triptease.com
Not sure if it is possible to integrate with short let sites but the serviced apartments and hotels are using it. They have it pop up on their property details: http://www.bymansley.com
If any short let managers are using it do let me know.