This is where we take a step back to a game that is actually well posed and for which Naive Decision Theory works fine - the programmers' game. Applying it there would go something like this:

• Create a probability distribution over the (potentially infinite) space of all possible programs that yours might be up against.
• Find the program which maximises the expected utility against that distribution of opponents.

... I believe this is called AIXI.

(Not exactly, but ...)