We know that in general that wearing a shardplate helm would help protect the wearer from emotional allomancy. However, often in the cosmere it's not a black and white matter of whether something can be done, or can't be done, but instead a matter of how powerful they need to be, or how much investiture is needed. For example, a normal misting cannot pierce a coppercloud, but an Inquisitor with additional bronze spikes can, and the Lord Ruler easily can. Likewise, a coppercloud can block a brass/zinc misting, but not emotional allomancy from the Lord Ruler. Metal cannot be easily effected by allomantic steel/iron if it pierces a person's body, and metalminds are extremely hard to push against, but Mist-fueled Vin was able to do so. So on, and so forth.
So, although we know that a shardplate helm would block a normal level of emotional allomancy, does it not stand to reason that someone as powerful as the Lord Ruler, whose soothing is shown to be many, many times over more powerful than normal base level emotional allomancy, would be able to have some effect on the person regardless of their shardplate?