Interesting discussion! The only time i have ever consciously asked a player to specifically load on the heel is when landing the front foot when hitting a neutral stance backhand. Too many players step onto the ball of the foot with this final step which basically puts the brakes on the stroke. A heel landing allows the heel to toe weight transfer that cheetah was referring to and lets the player 'roll' their body weight into the shot better. As for loading on the outside foot (during a forehand or wide backhand for example) I cant see any definitive evidence that elite players load on the heel rather than the whole foot or even the front of the foot - in terms of planting the foot a neutral weight or a more toe biased weighing would make more sense than a heel weighting as in that situation you don't want to continue the weight transfer in the direction of travel?
I would go and have a play about with this and experiment - but none of my players use their legs!!!