The big assumption here is that changing the strength curve, or how heavy the weight feels throughout the range of motion, from what naturally occurs with free weights is the right step. Accommodating resistance brings problems due to these reasons.
This would happen when your elbows are furthest away from your body on the bench press, which occurs at about the midpoint of the fullest possible range of motion. This already occurs with free weights, as long as you avoid too much range of motion.The process is the same with or without bands and chains.It seems then that as a powerlifter you should just practice with heavy singles.This creates less tension, the main stimulus for more muscle size and strength.Overloading the lockout then, which occurs with bands and chains, has your muscles work harder in a weak position. This also casts doubt upon partial rep training, another typical powerlifting technique.