You could switch your hand momentarily to the hood and shift from there. I assume you're not trying to shift while also maneuvering a tight hairpin and dialing it up to 400 watts all at the same time, so the little time it'll take shouldn't be an issue. Would also recommend being more proactive with your front shifting if you are not right now. As in, shift to the ring you'll be using before you get to the point where you need to make that shift, even if it also involves making a few rear shifts..