This list isn’t exhaustive. If I didn’t find someone, please add them to the comments.
Danny Aiken, long snapper for the New England Patriots, did a PG year at Fork Union where coach John Schuman taught Aiken the art of long snapping- a specialty that can earn a player a career. Aiken played collegiately at the Univerisity of Virginia and played with the Bills prior to coming to the Patriots.
Victor Cruz did a postgraduate year at Bridgton Academy in 2005. Un-hearalded coming out UMass, Cruz has developed in to a consumate slot receiver and one of Eli Manning’s primary targets.
Fellow New York Giant D.J. Ware did a post grad year Hargrave Military before attending the University of Georgia. Ware has been with Giants since 2007 and already has a Super Bowl ring.