Lockyer was always a 5/8th, but Bennett stuck him at Fullback until 5/8th became available (he debuted at 5/8th during an Origin series when the Broncos were depleted). It's testament to him that he thrived at fullback.

There are players from time to time who can excel in multiple positions - Greg Florimo & Jason Croker are (older) examples, but they're the exception in league, rather than the rule. I can't think of any top players in the last few years who have been able to serve as true "utility" players. Mannering at the Warriors is very versatile, but he's not quite at the same level that Lockyer is (and Croker and Florimo were).