Does adding baking soda to steam sweet potatoes in a pressure cooker help with caramelization? I recently discovered that adding baking soda to increase Ph in a pressure cooker promotes the Maillard reaction (see article, and this answer), and I want to try this with steamed sweet potatoes.

In both links, the food is submerged in the higher-Ph liquid. Could I achieve a similar result with steaming, by dissolving (possibly more) baking soda in the water, or does the food have to be in contact with the higher Ph environment?
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