What frequent flyer program offers their frequent flyers liberal upgrades on international flights?

Basically no frequent flyer program offer their frequent flyers upgrades on international flights - never mind liberal upgrades. On international flights the Business and First cabins are premium and they do not want the product diluted. However, during heavy travel periods, airlines will upgrade frequent flyers if they oversell the economy cabin. These upgrades are called OpUps - Operational Upgrades. They are done for operational reasons and not to reward a frequent flier.

The best chance for one of these is to fly on routes that are heavy on economy and low on business travellers and that use a plane such as the Boeing 757 rather than a Boeing 767/777/747 or Airbus A-330/340. An example of the is JFK-Dublin on Delta airlines.