Here's a list of former Pirates big-leaguers in the Pacific Coast League. I may have missed one or two; if so, let me know in the comments.
Colorado Springs Sky Sox (Brewers): Alex Presley was in late competition with Kirk Nieuwenhuis and fellow former Pirate Keon Broxton for one of two open Brewers outfield jobs, but he lost out and wound up at Triple-A. Presley was improbably successful during much of his career with the Pirates, but he hasn't done much since in two and a half seasons split between the Twins and Astros.