Implementation of surreal numbers for games
I don't have an answer to your question of whether the theory of Conway games has been used in building game-playing programs, but still you might be interested in the Combinatorial Game Suite, "an open-source program to aid research in combinatorial game theory" (which I first learned about here).  It includes an implementation of various standard operations on Conway games in canonical form, as well as a scripting language for describing new games.