There are a number of variants of frequentism, would you include hypothetical frequentism with a time-ordering? Namely, the idea that probability should be understood in terms of a hypothetical scenario where we could do an unending number of trials, such that if the frequency of some outcome in the first N trials is f_N (with 'first N' defined in terms of the time they occurred, not some other ordering), then the probability of that outcome would be defined as the limit of f_N as N approaches infinity. In this case I don't see why an infinite set of possible outcomes should be a problem--if each outcome has some finite probability, that means the number of trials with that specific outcome approaches infinity in the limit as the total number of trials approaches infinity, and the relative frequencies of any given pair of outcomes should approach the ratio of their probabilities according to QM laws.
How can the frequentist theory of probability be applied to a system, such as the H atom, which has an infinite set of possible outcomes for all energy level transitions? AG