What you're saying can be rephrased as follows. The prior probability measure on the space of (possibly rule-violating) Game of Life histories depends on f since it is the f-image of the Solomonoff measure. You are right. However, the dependence is as strong as the dependence of the Solomonoff measure on the choice of a universal Turing machine.

In other words, the complexity of the f you need to make G take a silly action is about the same as the complexity of the universal Turing machine you need to make G take the same action.