As i suggested to the rugby coach, one of the best methods to develop power using weights is to mix the heavy and lights stuff. i.e. 3 sets of 1-3 reps followed 5-10 reps at a much lower weight but literally at a decent pace.

Plyometrics are excellent too. There are all sorts of drills for sprinting and jumping which i am sure your coach will know of.

Just remember to keep it functional i.e. lunges and squats are better than leg extensions.

Chris