Does adding baking soda to steam sweet potatoes in a pressure cooker help with caramelization?
The need for contact is correct, but not really the issue here (else you could have pre-rubbed your potatoes). Both caramelization and Maillard reaction (which are different things) require rather high temperatures. The reason to use steaming as a technique is to not get up to these temperatures. In traditional steaming, you only get your food to 100°C. In a pressure cooker, you can steam the food quicker, but even in an old fashioned pressure cooker at sea level*, you never get above 120 Celsius, which is not enough for any browning - and that's a feature, not a bug. 

If you want Maillard reaction, or caramelization, you should choose a cooking method that can achieve that, such as roasting or pan-frying. If you are having trouble with pan-frying (e.g. having the outside burn while the center of the sweet potato chunks is still raw), you can first brown your food in a pan and then cook through with other methods such as braising, simmering or steaming. 

You can, in principle, add baking soda during a browning step, but for me, the change in taste is not worth it. Normal browning gives you plenty of tasty crust on its own. 

* Both elevation above the sea level and the use of modern electric pressure cookers mean less pressure, so lower temperature