Former Theater Center mainstays Kurt Rhoads and Nance Williamson have beenn longtime associates of the Hudson Valley Shakespeare Festival. So the two performers occasionally pop up in the pages of the New York Times whenever, it seems, a critic decides he needs to head upriver and see the incredible scenery on the Boscobel estate in Garrison, 50 miles north of Manhattan. This time, it’s Kurt who is involved in some unusual casting for a rarely done drama: He’s one-third of the title role of Shakespeare’s Pericles; an older and a younger actor play the other ages of the wandering king.