I think people of pretty much any body type can learn to stand, and do it effectively, including C/As. You just have to develop a sense of the proper gearing and cadence and then focus on technique. Too low a gear/too high a cadence and you'll exhaust yourself before you would have if you'd remained seated. Too high a gear/too low a cadence and you can find yourself grinding to a halt. Play around a bit. Once you get a better feel for the right gear for the hill, explore the other aspects of standing, like pulling up on the bars as you push down on the pedal, or consciously pulling up on the opposite pedal's upstroke (if your feet are connected to the pedals). When it works right, you can sometimes (not always) get to the top with less effort than you would sitting and spinning. And on those climbs where it takes more effort, at least you'll get to the top in less time.