Mariner:
The principle of specificity says that trying to model the demands of your event is always going to be give the best results. The reason we recommend a pack on the back is that it imposes a big strength and strength endurance load on the core and postural muscles that will directly affect your fatigue while climbing. If you notice the weight on your feel then you probably will benefit from training with heavy boots of ankle weights for at least some portion of your workouts.
Scott