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 …)