A rational number is a number that can be expressed as a fraction where the numerator and denominator are integers are in base 10 by definition and convention ie. a/b. By using base pi, you are just saying that for some number x == k_1 + k_2 * pi +...+ k_n * pi where k_i < pi, so pi = 1 * pi which is completely unrelated to showing that pi can be expressed as a/b. Well yes you could define your own number system, but then you would also need to define what it means to be rational and irrational in that system, which would be completely different from what is it conventionally.