Perhaps Voldemort didn't necessarily give Wormtail the hand for that reason. What I mean to say is that perhaps Voldemort wasn't conscious of the fact that Peter owed Harry a life debt, more than he was confident that Wormtail would at some point faulter in his loyalty to him in SOME situation, not necessarily in interaction with Harry. I think Voldemort was aware that Wormtail's 'loyalty' was one rooted in fear, not in a true loyalty to him and the cause, and he also knew that fear would cause Pettigrew to rethink his choices or have doubts about them at least. It was at that moment that he planned for the hand to play its role. The fact that it happened to be in an interaction with Harry may have been just slightly coincidental from Voldemort's perspective.