One can certainly compute the digits of pi, so that since (as non-intuitionists insist anyway) either the $n$th digit is even, or it is odd, we must have P($n$th digit is even) > P(axioms) or P($n$ digit is odd) > P(axioms).

I don't think that's valid - even if I know (P=1) that there is a fact-of-the-matter about whether the nth digit is even, if I don't have any information causally determined by whether the nth digit is even then I assign P(even) = P(odd) = ½. If I instead only believe with P=P(axioms) that a fact-of-the-matter exists, then I assign P(even) = P(odd) = ½ * P(axioms). Axioms ⇏ even. Axioms ⇒ (even or odd). P(axioms) = P(even or odd) = P(even)+P(odd) = (½ + ½) * P(axioms) = P(axioms), no problem. "A fact-of-the-matter exists for statement A" is (A or ¬A), and assuming that our axioms include Excluded Middle, P(A or ¬A) >= P(axioms).

Summary: P is about my knowledge; existence of a fact-of-the-matter is about, well, the fact-of-the-matter. As far as I can tell, you're confusing map and territory.