Yeah, the rear-derailleur shifts the un-tensioned side of the chain. So no matter how much force you're applying to the pedals, the rear will be able to shift.
Danno, the only thing is, when you're putting a lot of tension on the chain, it makes horrible popping sounds as the chain gets in line. I think it's bad news to shift under that much tension. It won't derail, but I
do think it puts undue stress on the cassette. It's often accompanied by manhandling the shifter, too. Force, as a rule, does not make machines run more smoothly.