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
An alkaline environment does indeed speed flavor reactions (enzymatic and non-enzymatic). You can create an alkaline environment with the addition of baking soda, as you point out. However there is a balance here that you have to consider. At some point, the flavor of the baking soda will become unpleasant. So, the reason a pressure cooker is used in the links you provided is that the high heat also speeds the flavor reactions of caramelization and Maillard. In this environment, the addition of baking soda speeds up the process even more. But one only needs about 0.5% baking soda for the effect in a pressure cooker. With lower temperatures (though you still need to reach the 140C necessary for Maillard, for example), and with the food not in direct contact, a much higher concentration of baking soda would be necessary. In this case, I am fairly certain you would taste it, and it would not be pleasant.
Another option for sweet potatoes, however, is to slow roast, or sous vide, for 2 to 4 hours (depending on method). Serious eats has a good post on this. It doesn't make use of an alkaline environment, rather it uses heat and time to activate enzymes in the potato that convert starches into sugars.