Just curious, what kind of sprint work are you doing? Distances?
I had an athlete 2 summers ago who was preparing for his first season of college ball, and we did 80 yd sprints. Used sleds, dragging and pushing, weight vests, various start positions such as laying prone, facing the opposite direction, etc. I'll be honest, I have no idea how big a rugby field even is, but if it's anything like football, the distances are usually short but very intense. You would obviously benefit from something that mimics the demands like intervals that match the rate of play. How much time is there between plays?