To the best of my knowledge, there is no definition of veteran other than in the military. And in the military, anyone who has served and been discharged (other than dishonorably) is a veteran. If we borrow that construction for Broadway, we would say that anyone who has a Broadway credit and lives to tell of it is a veteran. Of course, if you do a content analysis of the expression, you'll find that it is a creature of marketing, advertising, and PR folk, meaning that the term can be used fastly and loosely, in the sense that shows are routinely called Broadway bound, Broadway hits, Broadway blockbuster, etc etc etc. Since the original mention of the word in this thread seems to emanate from the great PR machine, anything goes.