Is the AI hardware separate from the cellular automaton or is it a part of it? Assuming the latter, we need to decide which degrees of freedom of the cellular automaton form the program of our AI. For example we can select a finite set of cells and allow setting their values arbitrarily. Then we need to specify our utility function. For example it can be a weighted sum of the number of gliders at different moments of time, or a maximum or whatever. However we need to make sure the expectation values converge. Then the "AI" is simply the assignment of values to the selected cells in the initial state which yields the maximal expect utility. Note though that if we're sure about the law governing the cellular automaton then there's no reason to use the Solomonoff semi-measure at all (except maybe as a prior for the initial state outside the selected cells). However if our idea of the way the cellular automaton works is only an "initial guess" then the expectation value is evaluated w.r.t. a stochastic process governed by a "deformed Solomonoff" semi-measure in which transitions illegal w.r.t. assumed cellular automaton law are suppressed by some factor 0 < p < 1 w.r.t. "pure" Solomonoff inference. Note that, contrary to the case of AIXI, I can only describe the measure of intelligence, I cannot constructively describe the agent maximizing this measure. This is unsurprising since building a real (bounded computing resources) AI is a very difficult problem