Of course it could be possible that you aren't allowed to access other people's essences. Some functions are limited to your own. Some bypass that using the tooltips and extract information available there. Others using inspection ..
However,
https://wow.gamepedia.com/World_of_W...API#Inspection
Shows very few documented inspection options and none to do with new data however, you may find something that will help via this file :
https://github.com/tomrus88/Blizzard...rDollFrame.lua which may highlight functions that are used during inspection to gain data and may be useable by addons.
Looking at
https://github.com/tomrus88/Blizzard...eEssenceUI.lua I can see no reference to a unit implying the player is the only unit the functions act on.